Moscow has called for an independent investigation by United Nations experts into allegations that the Syrian government carried out a deadly chemical weapons attack on the outskirts of Damascus.

The statement released by the Russian foreign ministry on Friday said Sergey Lavrov, the foreign minister, and the US secretary of state, John Kerry, had discussed the situation by telephone on Thursday and concluded that they had a "mutual interest" in calling for the UN investigation.

The statement said Russia had called for President Bashar al-Assad's embattled government to co-operate with an investigation, but questions remained about the willingness of the opposition, "which must secure safe access of the mission to the location of the incident".

Russia has been one of Assad's key allies in the international arena. Moscow has asserted that the attack was "a homemade rocket loaded with an unidentified chemical agent" and that it was probably a provocation by opposition forces intended to implicate the Syrian president.

Earlier, the UN secretary general, Ban Ki-moon, renewed his push for Syria to allow UN inspectors immediate access to the site of the alleged attack.

"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," he said at a diplomatic forum in Seoul.

Syria's government has offered no public response to UN calls for its team to inspect the site of the attack, in which opponents of Assad said 500 to more than 1,000 people died.

The White House has described itself as "appalled" by the reports of the death toll and the US held a flurry of diplomatic talks on Thursday to discuss possible action against the Syrian government.

Though it stressed it had still not yet seen conclusive proof of chemical weapon use, the US state department revealed that Kerry had held seven calls with his foreign counterparts on Thursday, and had taken part in a national security council meeting at the White House.

The British Foreign Office confirmed that Kerry had spoken to the foreign secretary, William Hague. A spokesman declined to comment on the contents of the call.

Washington is split over how to respond to the latest attack. Military leaders such as John Dempsey, the chairman of the joint chiefs of staff, have urged caution for fear of becoming further embroiled in a Middle East conflict when it is unclear whether the rebels would back US interests.

"Syria today is not about choosing between two sides but rather about choosing one among many sides," Dempsey said in the letter dated 19 August to Representative Eliot Engel. "It is my belief that the side we choose must be ready to promote their interests and ours when the balance shifts in their favour. Today, they are not."

Others, such as the UN ambassador Susan Rice, are thought to be keen to move beyond the limited supply of weapons to rebel forces sanctioned by President Barack Obama after the US first determined that the Syrian government used chemical weapons earlier this summer.

"Our red line was the use of chemical weapons. That was crossed a couple of months ago and the president took action," said the state department spokeswoman Jen Psaki. "If these [new] reports are true it would be an outrageous escalation in the use of chemical weapons by the regime, and there would be a range of further options for us to take."

She said Kerry spoke by phone with the leader of the Syrian national coalition, the UN secretary general and foreign ministers from France, Jordan, Qatar, Turkey and the EU.

"The president has ordered the intelligence community to urgently gather information. We are unable to determine conclusively chemical weapons use but we are doing everything possible to nail down the facts."

Psaki refused to detail or "inventory" previous arms supply measures, or speculate whether they had any effect in pressuring Assad.

The UN security council on Wednesday expressed "strong concern" and called for more clarity on the use of chemical weapons, but Russia and China insisted on the watering down of a tougher approach backed by the US, UK, France and 32 other governments that called on the UN investigative team already in Damascus to be allowed immediate access to the site of the attack, and to be granted greater latitude by the Syrian government to carry out their enquiries.

Assad's regime had previously allowed the UN team – led by a Swedish scientist, Åke Sellström – into the country, but limited its investigation into chemical weapons use to three sites.

Russia's foreign ministry spokesman Alexander Lukashevich said Syria's position on sending inspectors to the site of the reported attack should be respected but dismissed suggestions that Russia would object to such an investigation.

"The group of observers are already in place. Such a position was agreed upon in the UN security council. How can we object? We, quite the opposite, have an interest in the investigation into what happened happen objectively," he said.

"[The United Nations and Syria] have agreed on co-operation in three areas. If there is a need to achieve clarification in this case – and judging by everything, there is – then they need to agree," he told a news conference.

Moscow and Beijing have consistently backed Assad throughout the civil war, and the Russian foreign ministry on Wednesday accused rebels of staging the massacre to trigger intervention. China issued a statement saying it opposed the use of chemical weapons, but called for the UN team to "fully consult with the Syrian government and maintain an objective, impartial and professional stance, to ascertain what really happened".

The opposition Syrian National Coalition has said the Ghouta attack was just the latest in a series of chemical weapons atrocities, and joined international calls for Sellström's investigators to be given immediate access. The area was reported to be still under sustained Syrian army bombardment on Thursday.

"We cannot accept massacres, particularly involving the use of these extremely dangerous weapons. We're talking about mustard gas, sarin – things that remind us of the horrors of war," said the French foreign minister, Laurent Fabius. "The last time gas of this type was used on a massive scale was during the Iraq war, by Saddam Hussein."

France has frequently taken the lead in condemning Assad for atrocities, but has also stressed that its response would be in concert with its allies, leading to criticism of inaction within France. The headline of Le Monde's editorial on Thursday was: "Indignation is not enough".