Multiple large blasts have rocked Kenya's Westgate Mall where a hostage siege is in its third day.
Associated Press reporters on the scene Monday heard multiple blasts and a barrage of gunfire. Black and grey smoke rose up from the mall.
Security forces have been attempting to rescue an unknown number of hostages inside the mall held by al-Qaida-linked terrorists.
The White House is under pressure to ramp up counterterrorism action against al-Shabab in Somalia following the al-Qaida-linked group's deadly attack on an upscale Kenyan shopping mall that has killed and injured dozens, including Americans.
Republican lawmakers Sunday said the attack showed al-Qaida is growing in size and strength, belying the Obama administration's claims that it has grown weaker.
"They're not on the decline," said Sen. Tom Coburn of Oklahoma, the ranking Republican on the Homeland Security Committee, on CBS' "Face the Nation." `'They're on the rise, as you can see from Nairobi."
Al-Shabab militants launched their assault on Saturday, storming the mall with grenades and gunfire. Kenyan security forces launched a "major" assault late Sunday on the mall, where the militants were still holding an unknown number of hostages, trying to end the two-day standoff that had already killed at least 68 people.
Late on Sunday a military spokesman said that "most" of the hostages had been released. But a person with knowledge of the rescue operation told AP that no hostages had been released or rescued overnight. The person insisted on anonymity in order to talk about the rescue response.
Another indication no hostages had been freed: None appeared at the Oshwal Centre, a squat concrete structure that houses a Hindu temple just next to the mall that the Red Cross is using as a triage center. Medical workers attended to at least two wounded Kenyan soldiers there on Monday.
From Somalia, spokesman Sheik Ali Mohamud Rage for al-Shabab -- the militant group that claimed responsibility for the attack -- said in an audio file posted on a website that the hostage takers had been ordered to "take punitive action against the hostages" if force was used to try to rescue them.
Al-Shabab militants reacted angrily on Sunday to the helicopters hovering over the mall, and warned on Twitter that the Kenyan military action was endangering hostages.
State Department spokesman Marie Harf said five U.S. citizens were among the more than 175 injured, but no Americans were among those reported killed. Harf said U.S. law enforcement, military and civilian personnel in Nairobi are providing advice and assistance as requested by the Kenyan authorities.
U.S. counterterrorism officials throughout the Obama administration have debated whether to target the Somalia-based rebel group more directly, especially after it merged with al-Qaida in early 2012. But U.S. action has been limited to the occasional drone strike or raid when a particularly high-value al-Qaida target comes into view, while relying primarily on assisting Somali and African peacekeeping forces to carry out the day-to-day fight.
That decision was partly driven by the fear that directly targeting al-Shabab would spur the group to expand its own target list, striking at U.S. diplomatic posts overseas and calling on members of the Somali diaspora inside the U.S. to carry out attacks, according to multiple current and former U.S. counterterrorism officials. They all spoke on condition of anonymity because they were not authorized to discuss publicly internal policy decisions.
A White House official said Sunday that the administration had taken a "balanced approach."
"It's not a question of either direct action or playing a supporting role," National Security Council spokesman Jonathan Lalley said by email Sunday. "Our approach has been to work to enable and support African partners," as well as prosecuting some al-Shabab members and supporters, he said.
"The U.S. military has also taken direct action in Somalia against members of al-Qaida -- some of them members of al-Shabab -- engaged in efforts to carry out terrorist attacks against the United States and U.S. interests," Lalley said.
But that effort in Somalia pales next to, say, the hundreds of U.S. drone strikes against militants in Yemen and Pakistan during the Obama administration.
The Somali rebel group has similarly limited its own target list to Somali officials or troops, and African Union peacekeeping troops, to avoid drawing the U.S. counterterrorism machine into a full-fledged fight, the U.S. officials say. Though headed by hard-core Islamist militants, al-Shabab's more moderate membership has successfully argued to keep the group focused on overthrowing the U.S.-backed Somali government, rather than taking on the mantle of al-Qaida's larger war with the west.
The group did claim responsibility for twin suicide bombings in Uganda in 2010 that killed more than 70 people, but that was seen as a reaction to Uganda providing the bulk of African peacekeeping forces in Somalia.
Similarly, al-Shabab said this weekend's attack was in retribution for Kenyan forces' 2011 push into Somalia.
"You reach the population who says the cost we're bearing for this operation in Somalia is too much," said al-Shabab expert Daveed Gartenstein-Ross, of the Washington-based Foundation for Defense of Democracies. "From Shabab's calculus, they may well think it's worth inflicting a heavy cost on Kenya," even if it draws U.S. ire.
But the scale and technical sophistication of the Nairobi attack could signal a change in al-Shabab's aspirations, according to Rep. Peter King, R-N.Y., possibly increasing the group's direct threat to the United States. King said the State Department had not initially wanted to declare al-Shabab a terrorist organization because it saw the group focusing on tribal issues within Somalia. It was declared a terror organization in 2008.
"Now, we see, by attacking into Kenya they certainly have an international dimension to them," King said Sunday on ABC's "This Week." `'We're talking about very significant terrorist groups here which are showing a capacity to attack outside of their borders and actually recruit people from here in the United States," said King, who serves on the House Intelligence Committee.
The attack is a recruiting and fundraising shot in the arm for al-Shabab's leader, Mukhtar Abu Zubeyr, who is working to consolidate power after a year spent eliminating rivals, according to Raffaello Pantucci, who has studied the group for West Point's Combating Terrorism Center.
"It's a trifecta for the group," Pantucci said in an interview Sunday. "It brings attention, causes chaos and is successful."
Leaving the violence unanswered could be a further boon for the organization.
Up until now, President Barack Obama secretly has authorized only two commando raids and at least two drone strikes against the al-Qaida linked terrorists in Somalia, while a small U.S. special operations team has advised African peacekeeping troops, as well as helping build a small elite Somali counterterrorism force, according to two former U.S. military officials familiar with the operations.
Two former U.S. counterterrorism officials say the preference has always been to meet specific incidents with a specific response but to avoid getting too deeply involved in the continent of Africa.
"The `don't expand the fight' argument has always won," one said.
They said the number of western citizens among the dead and injured in the weekend incident may change the U.S. calculation.
-- Jason Straziuso, Rodney Muhumuza and Abdi Guled contributed to this report.