Russia has said it told Syria to cooperate with UN experts investigating reports of a deadly chemical weapons attack that is believed to have killed hundreds, and called on rebels to allow access to the area.

Foreign Minister Sergei Lavrov told his US counterpart John Kerry in a phone call that immediately after the reports first emerged about the attack on Wednesday, the "Russian side called on the Syrian government to cooperate with the UN chemical experts," the foreign ministry said in a statement on Friday.

"It is now up to the opposition to ensure safe access for the mission to the site of the alleged incident," the statement from Syria's strongest ally said.

It said both Lavrov and Kerry agreed on the need for an "objective investigation" into what happened.

The Syrian opposition has alleged that President Bashar al-Assad's government on Wednesday fired rockets with chemical warheads into the Damascus suburbs of Arbeen, Zamalka and Ein Tarma. The attack reportedly left hundreds dead.

The opposition said on Friday that it would ensure the safety of the UN chemical weapons inspectors in areas under their control and stressed that it was critical that the inspectors reached the site of the alleged attack within 48 hours.

Meanwhile China, another Syrian ally, has said that no side should rush to pre-judge the results of any probe by UN chemical weapons experts in Syria.

"The UN inspection team has already gone to Syria to start their probe, and [China] hopes and believes they can fully consult with the Syrian government to ensure the smooth progress of the investigation work," China's Foreign Ministry said.

Moscow and Beijing have both vetoed previous Western efforts to impose UN penalties on Syrian government.

Joint UN-Arab League envoy Lakhdar Brahimi, meanwhile, said that the alleged attack showed the urgency of Syria peace talks.

"Brahimi believes that the alleged chemical weapon attack in Syria this week should speed up work towards an international peace conference", his spokeswoman, Khawla Mattar said.

Brahimi has stepped up contacts with senior US and Russian officials due to meet in The Hague next Wednesday and may hold talks with them subsequently, probably in Geneva, she said.

US President Barack Obama said in an interview on Friday that the international community needed to find out more about whether chemical weapons have been used in Syria, and called on the Assad government to allow a full investigation.

"What we've seen indicates that this is clearly a big event, of grave concern," Obama said in a television interview with CNN.

UN chief calls for probe

Earlier, the UN Secretary-General Ban Ki-moon has asked the Syrian government to allow UN inspectors to investigate "without delay" the latest alleged chemical attack in the country's civil war.

Ban said that the use of chemical weapons in Syria would constitute a "crime against humanity", and warned Bashar al-Assad's government of "serious consequences" if their use was proven.

"Any use of chemical weapons anywhere, by anybody, under any circumstances, would violate international law," he said.

"This is a grave challenge to the entire international community - and to our common humanity, especially considering it occurred when the United Nations expert mission is in the country," Ban said.

"I can think of no good reason why any party, either government or opposition forces - would decline this opportunity to get to the truth of the matter," the UN chief told a diplomatic forum in the South Korean capital, Seoul.

Meanwhile, activists said government warplanes hit areas where the alleged chemical attack took place again on Thursday.

Navi Pillay, the UN high commissioner for human rights, echoed Ban's sentiment and said the gas attack allegations were "exceptionally grave".

Syria's government, which has denied using chemical weapons, has offered no public response to the UN calls for its team to inspect the site of the attack.