There are 329 parliament seats at stake, with nearly 7,000 candidates from dozens of political alliances.
The four troops killed in the Niger fought with honor, according to a Pentagon account.
Iraqi forces in coordination with U.S.-backed Syrian forces have captured five senior Islamic State group leaders.
U.S. and coalition forces have increased airstrikes and artillery fire against ISIS fighters in support of Operation Roundup.
The U.S. deactivated the headquarters of the coalition's land component in Iraq.
In the past week, the coalition conducted 14 airstrikes against ISIS fighters in defensive fighting positions in Syria.
Commanders are seeing new pockets of ISIS movement in a handful of areas in Syria, which has led to an uptick in airstrikes.
The lack of access to the town of Douma by inspectors from a watchdog has left questions about the April 7 attack unanswered.
The White House said the U.S. troop presence in Syria is coming to a "rapid end."
Gen. Joseph Votel wants to keep U.S. troops in Syria; President Donald Trump wants them out.