John Bacon

USA TODAY

The United States and Britain said Sunday they are considering sanctions against Syria and even Russia as conditions deteriorate in the war-torn Syrian city of Aleppo.

Secretary of State John Kerry and British Foreign Secretary Boris Johnson, speaking at a gathering of diplomats from almost a dozen nations in London, stressed the need for a political solution to the brutal Syrian civil war now in its sixth year.

Almost 300,000 people in Aleppo, which remains a stronghold of U.S.-backed rebels, have been virtually held hostage and nearly starved by unrelenting bombing.

"All of us are more than concerned and deeply, deeply disturbed by and outraged by what is happening in Aleppo," Kerry said. "This is the largest humanitarian disaster since World War II."

Kerry said the horror "could stop tomorrow morning" if Russia — Syria's staunch ally — and the government of Syrian President Bashar Assad acted with decency. Crimes against humanity are a daily occurrence in Syria, he said. The U.S., Britain and France want Russia and Assad's government investigated for war crimes.

"We are discussing using every mechanism available to us, but I haven’t seen a big appetite in Europe of people to go to war," Kerry said. "And let me make it clear: We are considering additional sanctions."

Johnson said more than 10,000 rebels remain armed in Aleppo, making it folly for the Assad regime and its backers to believe they can ultimately claim a military victory. A cease-fire, he said, would be in the long-term, best interests of the Russian people as well as the Syrian people.

"It is vital that we keep that pressure up — and there are a lot of measures we're proposing — to do with extra sanctions on the Syrian regime and their supporters," Johnson said. "Measures to bring those responsible for war crimes to the International Criminal Court."

Little can be done, however, without the participation of Russia and others who back Assad, Johnson acknowledged.

Hundreds of civilians have died in the air campaign since the collapse of a cease-fire brokered between the United States and Russia. The bombing, which Russia says is aimed at ridding the city of "terrorists," has targeted hospitals and civilian neighborhoods, according to the U.S. State Department.

Russia has deployed advanced anti-aircraft missiles to Syria and warned the U.S. that it would use them against any aircraft conducting strikes on territory controlled by the Syrian government.

"I’m not going to pretend ... there is some magic solution for this appalling slaughter," Johnson said. "The real answer, I’m afraid, lies with those who are perpetrating it, and that is overwhelmingly the Assad regime and its puppeteers in the form of the Russians and, indeed, the Iranians."