LONDON (AP/AFP/REUTERS) - The British government says it welcomes and agrees with the US assessment that Syrian President Bashar Assad has used chemical weapons against opposition forces.

British Foreign Secretary William Hague said on Friday that the UK agrees with the US assessment that chemical weapons, including sarin gas, have been used.

The UK and France both presented evidence of chemical weapons use in Syria to the United Nations, urging that investigators be allowed unfettered access to sites on the ground.

Mr Hague said in a statement on Friday that the crisis in Syria demands a strong and coordinated response, adding that the international community must "be prepared to do more".

The United Nations on Thursday raised the overall death toll in Syria's civil war to nearly 93,000.

Syria on Friday slammed Washington's claims that it had used chemical weapons in its fight against rebels, accusing the United States of "lies" based on "fabricated information".

"The White House published a statement full of lies about the use of chemical weapons in Syria, based on fabricated information, through which it is trying to hold the Syrian government responsible for such use," state news agency SANA quoted a foreign ministry official as saying.

The official said the US claims came "after reports affirming that armed terrorist groups active in Syria are in possession of deadly chemical weapons and the technology necessary to make them".

Washington said on Thursday there was evidence that the Syrian government had used chemical weapons including sarin nerve gas in attacks that killed up to 150 people.

The United States is considering imposing a no-fly zone in Syria, in what would be its first direct military intervention of the two-year-old civil war, Western diplomats said on Friday, after the White House said Syria had crossed a "red line" by using nerve gas.

The US administration also said it would now provide "military support" to rebels, a decision condemned by Damascus.

"The American decision to arm armed terrorist groups demonstrates... the direct involvement of the United States in the Syrian bloodbath," the official said.

"This raises serious questions about their good faith when it comes to finding a political solution in Syria," the official added.

Washington and Moscow - which is allied with the Syrian regime - have been spearheading efforts to organise a conference in Geneva to find a political solution to the Syrian conflict.

Imposing a no-fly zone would require the US to destroy Syria's air defences, entering the civil war with the sort of action that NATO used to help topple Muammar Gaddafi in Libya two years ago.