U.S. Secretary of State John Kerry is accusing the Syrian regime of "systematically destroying evidence" in an area believed to be the site of a chemical weapons attack, saying the shelling of suspect sites is "not the behaviour of a government that has nothing to hide."

Kerry said he spoke last week to Syria's foreign minister, and urged him to offer immediate access and full transparency to UN inspectors seeking to visit the site of the attack near Damascus.

"Instead, for five days, the Syrian regime refused to allow the UN investigators access to the site of the attack that would allegedly exonerate them," Kerry said.

The UN team's stated mission is only to determine whether chemical weapons were used, but not to determine who used them. The United States has previously noted that it strongly suspects that President Bashar al-Assad's regime was behind the Aug. 21 attack near Damascus, and Kerry ramped up that message on Monday.

"What is before us today is real," Kerry said. "And it is compelling."

Kerry noted that it's known that the Syrian regime "maintains custody" of chemical weapons and has been "determined to clear the opposition" from the areas where the attacks took place.

The secretary of state said the reported number of victims, the reported symptoms of the people killed and injured and firsthand accounts from humanitarian organizations "all strongly indicate that everything these images are already screaming at us is real — that chemical weapons were used in Syria."

That suspicion is supported by the international aid group Doctors Without Borders, which reported that 355 people were killed in an artillery attack that also included the purported use of a toxic chemical weapon. Its numbers are also consistent with those of Syrian activists and opposition leaders, who have said that between 322 and 1,300 people were killed in the alleged chemical attack.

Kerry also noted that the U.S. has "additional information" about the attack that is being reviewed with partners. He didn't offer specifics, but said the information will be provided in the days ahead.

UN investigators in Damascus

UN investigators were eventually permitted access to the sites, but delays continued earlier today, when inspectors were held up by sniper fire near Damascus. The UN inspection team nevertheless proceeded towards a rebel-controlled western suburb to meet with doctors and victims of last week's purported chemical attack.

The UN inspectors have since returned to their hotel in central Damascus, after having spent three hours collecting samples from the site in the outskirts of the capital.

UN Secretary General Ban Ki-moon said the team visited two hospitals and interviewed witnesses, survivors and doctors while collecting samples around the Rawda Mosque. Ban said the team was committed to completing its mission despite "very difficult circumstances," referring to their brush with sniper fire.

Angela Kane, the UN under-secretary general for disarmament, briefed him on the incident.

"I have instructed Angela Kane to register a strong complaint to the Syrian government and authorities of opposition forces so that this will never happen and the safety and security of the investigation teams will be secured from tomorrow," Ban said.

Although there were no injuries from the assault on the UN inspectors, CBC’s Melissa Kent reported that "the lead vehicle was so badly damaged … the team had to turn around and return to the government checkpoint to exchange vehicles."

The Syrian government and opposition fighters continue to trade blame over who was responsible for the sniper attack.

Mortar attack At least two mortar bombs struck a wealthy district of central Damascus on Monday, in the same area where a team of United Nations inspectors are staying as they investigate alleged chemical weapons attacks. Syrian state media said the mortar bombs were locally made and fired by "terrorists," its term for rebels. SANA state news agency said three people were wounded. UN inspectors had reportedly already left their hotel to visit sites of the alleged chemical weapons strike on the outskirts of Damascus last week. — Reuters

Syria President Assad had earlier pledged that a ceasefire would be in place while the investigators worked.

A UN chemical weapons expert stands at one of the sites of an alleged poison gas attack in the southwestern Damascus suburb of Mouadamiya. (Abo Alnour Alhaji/Reuters)

Intimidation tactics are nothing new to UN weapons investigators in a foreign land, according to former UN arms investigator Tim Trevan.

"Clearly this is intimidation against the team," Trevan told CBC News from Washington. "It's something teams have to deal with, confrontation situations like this one."

Trevan added that although it's difficult to say whether the sniper was with the opposition or government forces, "the suspicion must be that this is the Syrian regime."

"They want to show the international community they're willing to go ahead and co-operate, and at the same time ensure it doesn't happen by subjecting the convoy to fire, making the inspection site itself too dangerous to visit," Trevan said.

Canada's response

Canadian Foreign Affairs Minister John Baird reiterated that Canada is still committed to a peaceful solution to the violence.

"Canada believes the only way to help the bloodshed in Syria is through a political solution," he told reporters today in Ottawa. "However, we understand that this solution is becoming more and more difficult as the crisis enters a very dangerous new phase."

Baird has been speaking with his French and British counterparts about the Syria situation and expressed a shared "outrage" over reports about possible chemical attacks, according to a spokesperson from his office.

Syrian-Canadians have been calling on Ottawa to participate in an international action to stop the violence and Baird has acknowledged that the delay in allowing UN inspectors to do their work will likely impair their investigation.

Baird also heaped more diplomatic pressure on Russia on Monday, criticizing the nation for using its veto on the UN Security Council to block resolutions designed to punish Syria with sanctions.

"The complete obfuscation of Russia, the Security Council and the United Nations needs to end, and they need to become part of the solution," Baird said.

'Too late to be credible'

Although Syria has said that a UN team was welcome to visit the site, a senior White House official dismissed the deal with inspectors as "too late to be credible."

British Foreign Secretary William Hague added that it was likely that artillery fire at the site would have destroyed much of the evidence.

Angela Kane, the UN high representative for disarmament affairs, watches a convoy of UN inspectors heading to the scene of an alleged chemical weapons strike in a rebel-held area in Syria. (Khaled al-Hariri /Reuters)

Meanwhile, a defiant Assad claimed foreign leaders were making excuses so they could intervene militarily in Syria, telling a Russian newspaper the accusations that his troops used chemical weapons were "politically motivated."

"This is nonsense," Assad was quoted as saying in the interview with Russia's Izvestia daily. "First they level the accusations, and only then they start collecting evidence."

The UN team's conclusions could have a dramatic impact on the trajectory of Syria's civil war as France, Britain, Israel and some U.S. congressmen urge swift military action against Assad's regime.

Meanwhile, Germany has indicated willingness for the first time to support a possible military response in Syria.

Chancellor Angela Merkel said Syria "must be punished" if UN inspectors are able to confirm the use of chemical weapons by Assad's forces.

Turkey, another of Assad's harshest critics, said it would back an international coalition to move against Assad if sanctions against the government fail.

However, Syria still has a staunch ally in Russia.

Russian Foreign Minister Sergei Lavrov accused Western powers of having hawkish tendencies despite a lack of evidence to show the Syrian government was behind the purported chemical attack.

The countries calling for action "cannot provide evidence" of such an attack, Lavrov said in a televised conference on Monday, adding that talk of military action is undermining efforts to resolve the crisis peacefully.