The United Nations team investigating an alleged chemical weapons attack in Syria was fired upon Monday by snipers, casting doubt on the Assad regime's ability to provide safe passage. 

A U.N. statement released Monday morning said a U.N. vehicle was "deliberately shot at multiple times by unidentified snipers in the buffer zone area." The damage to the vehicle forced the team to return to the government check-point and replace the vehicle. 

"It has to be stressed again that all sides need to extend their cooperation so that the team can safely carry out their important work," the U.N. statement said. 

The Syrian government accused the opposition of firing at the U.N. team, and claims Syrian forces were trying to provide safety for the investigators. The Syrian government said the U.N. team was subjected to fire by "terrorist gangs" while entering the Damascus suburb of Moadamiyeh west of Damascus, one of the areas that the opposition says were targeted by toxic gas in last week's attack. 

But western governments had earlier cast doubt on the suggestion -- made earlier by the Russians -- that the U.N. team might be at risk from rebel attacks. 

State Department spokeswoman Jen Psaki said late last week, according to The Washington Post, that  "the only hindrance to the U.N. investigative team having unfettered access to all sites where there are credible reports of chemical weapons use is the Syrian regime." 

News of the sniper attack came only a few hours after an Associated Press photographer saw the team members wearing body armor leaving their hotel in Damascus in seven SUVs, headed to the site of the alleged attack. 

The photographer said U.N. disarmament chief Angela Kane saw them off as they left but did not go with them. 

Nearly an hour before the team left, several mortar shells fell about 700 yards from their hotel, wounding three people. One of the shells struck a mosque and damaged its minaret, according to an AP reporter on the scene. 

World leaders have suggested that an international response to the attack was likely. 

The United States has said that there is little doubt that Assad's regime was responsible for the attack on Aug. 21 in the capital's eastern suburbs. 

The group Doctors Without Borders said 355 people were killed in the artillery barrage by regime forces that included the use of toxic gas. 

President Bashar Assad denied in remarks published Monday that his troops used chemical weapons during the fighting in the rebel-held suburbs.
Wassim al-Ahmad, a member of the Moadamiyeh local council, said five U.N. investigators eventually arrived at a makeshift hospital in the suburb, where doctors and about 100 people still with symptoms from the alleged chemical attack were brought in to meet with the U.N. team. 

The Britain-based Syrian Observatory for Human Rights says the eastern suburbs have witnessed a wide army offensive over the last week, but have been relatively quiet since Sunday night. 

Mohammed Abdullah, an activist in the eastern suburb of Saqba, said the U.N. is expected to visit the rebel-held area on Monday and will be under the protection of the Islam Brigade, which has thousands of fighters in the area. 

Syrian activists and opposition leaders have said that between 322 and 1,300 people were killed in the alleged chemical attack. 

The Associated Press contributed to this report.