By Lesley Wroughton

LONDON (Reuters) - Britain and the United States said on Sunday they were considering imposing additional sanctions on Syrian President Bashar al-Assad and his supporters for their actions in Syria's war.

British Foreign Secretary Boris Johnson and U.S. Secretary of State John Kerry, speaking after briefing allies in London on a new diplomatic initiative on Syria, also called on Russia and Iran to agree to a new ceasefire.

The threat of more sanctions on Syria came before a European Union summit on Thursday and Friday to discuss sanctions against Russia.

"There's a lot of measures we're proposing to do with extra sanctions on the Syrian regime and their supporters, measures to bring those responsible for war crimes to the International Criminal Court," Johnson told reporters after talks he convened with his U.S counterpart and allies on Syria.

"These things will eventually come to bite the perpetrators of these crimes and they should think about it now," said Johnson, adding there was no appetite in Europe for going to war in Syria.

He said it was "highly dubious" that Syrian government forces backed by Russia were capable of retaking the city of Aleppo or winning the war, and called on Russia and Iran to show leadership by agreeing to a ceasefire.

"It is up to them to show mercy, show mercy to those people in that city and get the ceasefire going," he added.

He spoke alongside Kerry, who briefed European and other allies on a new diplomatic initiative involving Russia and a group of Middle Eastern nations aimed at ending the fighting in Syria. The first round of talks in the Swiss city of Lausanne failed to agree on a strategy for ending the violence soon.

Kerry confirmed the U.S. was considering additional sanctions over Syria, but did not name Russia as a target.

Western powers have accused Russia and Syria of committing atrocities by bombing hospitals, killing civilians and preventing medical evacuations, as well as targeting an aid convoy with the loss of around 20 lives.

Syria and Russia say they are only targeting militants in Aleppo and accuse the United States of breaking the ceasefire by bombing scores of Syrian troops fighting Islamic State insurgents, over which the United States has expressed regret.

"We are considering additional sanctions and we are also making clear that President (Barack) Obama has not taken any options off the table at this point in time," Kerry said.

Washington suspended bilateral discussions with Moscow over Syria following two attempts at implementing a ceasefire and growing tensions in their relationship.

With the U.S. presidential election less than a month away and Obama unwilling to assume a deeper role in the Syrian war, Kerry is trying to build a broader dialogue involving key regional players in the Syrian conflict.

The U.S. and its allies have urged Moscow to use its influence with the Syrian government to end the bombardment of Aleppo.

"There is some work to be done over the course of the next couple of days which might, or one might hope, open the door of possibility to an actual cessation," Kerry said.

"It's hard, and it's hard because there are still deep beliefs in a lot of people that Russia is simply pursuing a Grozny solution in Aleppo and is not prepared to truly engage in any way."

Moscow all but destroyed Grozny, the capital of Russia's Chechnya region, during its 1999-2000 war against Islamist separatists there.

The United States first imposed sanctions against Syrian government officials in March 2011 shortly after the uprising that led to the civil war.

In 2013, Washington eased some of the restrictions to allow for reconstruction in opposition-held areas.

Both the EU and the United States have already imposed economic and other sanctions on Russia for its seizure of the Crimean peninsula from Ukraine in 2014, and for its support for pro-Russian rebels in eastern Ukraine.

(Reporting by Lesley Wroughton; Editing by Tom Heneghan)