Sniper fire struck a U.N. chemical weapons inspection team's vehicle in Syria several times Monday, the United Nations said.
The inspection team "returned safely back to the government checkpoint," a U.N. statement said. There were no reports of injuries.
The statement said the team was replacing the vehicle and would return to the areas, CNN reported.
The inspectors plan to examine the site of last week's attack, the Damascus suburb of Jobar, U.N. Secretary-General Ban Ki-moon said. The Syrian opposition said more than 1,100 civilians were killed by nerve gas.
For days, the Syrian government would not let the inspectors near the site and inspectors said they feared chemical evidence may have dissipated.
On Sunday, the Syrian government agreed to grant inspectors full access.
"Every hour counts. We cannot afford any more delays. We have all seen the horrifying images on our television screens and through social media. Clearly, this was a major and terrible incident," Ban said. "We owe it to the families of the victims to act."
CNN reported there was an explosion near the site the inspectors plan to visit. Some witnesses said it was caused by incoming ordnance.
On Monday, Syrian President Bashar Assad repeated his statement that his army had nothing to do with the use of poison gas.
"The area of the claimed attack is in contiguity with the Syrian Army positions, so how is it possible that any country would use chemical weapons in an area where its own forces are located?" he said in an interview with Russian newspaper Izvestia.
He also warned that an attack on his country would "face failure."
"Statements made by politicians in the U.S. [and other countries] is an insult to common sense, a disregard of people's public opinion. [The allegations] are nonsense: first indict and only then collect evidence?" Assad said in the interview published Monday.
U.S. Defense Secretary Chuck Hagel said U.S. forces were "prepared to exercise whatever option" President Barack Obama ordered.
Obama warned a year ago that Syrian use of chemical weapons would cross a "red line."
If the United States strikes or invades Syria, it will "face failure, as in all previous wars they conducted from Vietnam to the present day," Assad warned. "Yes, great powers can unleash a war, but can they win?"
Asked what his message to the world is, Assad said, "If you insist I send a message to the world, I say if someone wants to turn Syria into a puppet of the West, it will not. We are an independent country and will fight against terrorists and will be free to build relations with those countries with whom we wish, for the sake of the Syrian people."
British Foreign Secretary William Hague said Britain, the United States and their allies were militarily poised to deliver a harsh military message to Assad after Wednesday's alleged nerve gas attack on Jobar.
"We cannot in the 21st century allow the idea that chemical weapons can be used with impunity, that people can be killed in this way and that there are no consequences for it," Hague said.
Syrian ally Russia has accused rebels of staging the attack.
Obama spoke with British Prime Minister David Cameron, French President Francois Hollande and German Chancellor Angela Merkel during the weekend. All the leaders agreed on the need for a "serious response" to the alleged attack, officials said.
It was not immediately clear whether France and Germany would participate in any military action against Syria.
Russia cautioned against military action without U.N. approval, saying countries wouldn't want to repeat the 2003 U.S.-British-led coalition "mistakes" in the invasion of Iraq to remove weapons of mass destruction that were never found.
Iran predicted "harsh consequences" if the United States intervened against the Iranian ally.
Israeli President Shimon Peres called for an international effort to "take out" Syrian chemical weapons.
A senior Obama administration official said in a statement Sunday the United States and allies had "very little doubt" Assad's military forces used chemical weapons against civilians.
The most likely military option involves long-range cruise-missile strikes from a U.S. destroyer and other military watercraft in the Mediterranean Sea because the Syrian air force is considered strong enough to shoot down enemy jets, officials told several news organizations.
A British nuclear-powered submarine is in the region and a number of British warships are traveling to the Mediterranean for exercises, the Telegraph reported.