Syria: US and UK call for action to stop loss of life
More must be done to stop the "outrageous" loss of life in Syria, William Hague has said after talks with his US counterpart in Washington.
Diplomatic efforts would be intensified amid reports that the Syrian government was planning "new assaults" on rebel groups, the UK Foreign Secretary said.
But he and US Secretary of State John Kerry would not be drawn on discussions about arming the Syrian opposition.
David Cameron is to discuss the crisis with Vladimir Putin on Sunday.
The meeting will take place on the eve of the G8 summit in Northern Ireland, set to be dominated by the Syrian conflict.
Russia has been at loggerheads with the US and UK over the situation in Syria amid efforts to bring both sides in the two-year conflict, in which 80,000 people have been killed, to the negotiating table.
Mr Hague said more had to be done to save lives in Syria and the international community must be "strong, co-ordinated and determined" in its approach to what he described as the "most urgent crisis" facing the world.
He said the level of repression by the government of President Bashar al-Assad and the suffering it had caused "beggared belief", describing its actions as a "moral outrage" and a "grave threat to the wider region".
While the UK's focus remained on a diplomatic solution, he said the regime's actions were making this "more difficult", adding that reports of the use of chemical weapons were credible.
Mr Kerry said the US was doing all it could to support the opposition to "change the (military) balance on the ground" but indicated that no agreement had been reached on the issue of whether opposition forces should be provided with arms.
Referring to the alleged use of chemical weapons, he said the Syrian government's actions had "challenged anybody's values and standards of human behaviour".
More than 80,000 people have been killed and 1.6 million have fled to neighbouring countries since the uprising against President Assad began in March 2011, according to the UN.
The UK and France have led calls for more support for the official opposition in Syria - although they have insisted the EU's decision to lift the arms embargo last month, heavily criticised by Russia, is not a prelude to the immediate sending of arms by them.
Critics have said such a move could increase the bloodshed and result in weapons falling into the hands of extremists.
Mr Cameron has said Sunday's meeting with the Russian president is designed to "crank up the pressure" on all sides in the conflict. Speaking in Parliament, he said all sides wanted an end to violence but there were disagreements on how to do this.
Speaking on Wednesday, Russian Foreign Minister Sergei Lavrov said any transitional body set up in Syria in the future must be "based on consent between the government and the opposition".