US President Barack Obama asks his staff to put forward options to consider how Washington might respond to the Russian-backed regime's assault in Syria.
The US is close to suspending talks with Russia on a ceasefire in war-torn Syria, US Secretary of State John Kerry said on Thursday, as the Kremlin vowed to continue with an offensive on the city of Aleppo.
"We are on the verge of suspending the discussion because it is irrational in the context of the kind of bombing taking place to be sitting there trying to take things seriously," Kerry told a public policy conference in Washington.
"It is one of those moments where we are going to have to pursue other alternatives," he added.
Kerry spoke to Russian Foreign Minister Sergei Lavrov by phone on Thursday about the "fragility" of a ceasefire agreement on Syria, the US State Department spokesperson John Kirby said.
"The secretary did speak to Foreign Minister Lavrov this morning ... about the situation in Aleppo, about the fragility of the arrangement that we struck earlier this month in Geneva," Kirby told a briefing. "We are prepared to enact that kind of a suspension," he added.
Russia and Syrian regime launched a campaign to recapture the opposition-held sector of Aleppo this month, abandoning a ceasefire a week after it took effect to embark on what could be the biggest battle of a nearly six-year war.
The regime forces made a significant advance, capturing the Handarat refugee camp a few kilometers (miles) north of the city. They had briefly seized it on Saturday, before losing it again in a counter attack by opposition forces.
Opposition fighters have launched an advance of their own near the central city of Hama, where they said they made gains on Thursday.
Russia vows continued support for Syrian regime
Kremlin spokesman Dmitry Peskov, meanwhile, said Russia would continue the operation of its air force in support of the Syrian regime's forces.
Peskov said Washington was to blame for the fighting, by failing to meet an obligation to separate "moderate" opposition fighters from those he called terrorists.
"In general, we express regret at the rather non-constructive nature of the rhetoric voiced by Washington in the past days."
EU decries Aleppo air strikes
The European Union accuses Russia of torpedoing diplomacy to pursue military victory in Aleppo, and says Moscow and Syrian regime are targeting civilians, hospitals and aid workers to break the will of 250,000 people living under siege in the city.
EU foreign affairs chief Federica Mogherini called the air strikes in Aleppo a "massacre" and said European governments were considering their response.
Syria's UN Ambassador Bashar Ja'afari rejected accusations that the regime was killing civilians.
Russia and the Syrian regime say they are targeting only militants.
Obama, Merkel condemn assault
US President Barack Obama spoke by phone on Thursday with German Chancellor Angela Merkel and the two leaders "strongly condemned the barbarous Russian and Syrian regime air strikes against eastern Aleppo," the White House said in a statement.
"They agreed Russia and the Syrian regime bear special responsibility for ending the fighting in Syria and granting the UN humanitarian access to besieged and hard to reach areas in Syria," the statement said.
Washington considering response to asssault
US Deputy Secretary of State Antony Blinken told his country's lawmakers that President Barack Obama had asked staff to look at how Washington might respond to the Russian-backed regime assault in Syria.
"The president has asked all of the agencies to put forward options, some familiar, some new, that we are very actively reviewing," Blinken said, adding that officials would "work through these in the days ahead."
Possible responses include allowing Gulf allies to supply opposition fighters with more sophisticated weapons, or carrying out a US air strike on a Syrian regime air base, viewed as less likely because of the potential for causing Russian casualties, US officials told Reuters, speaking on condition of anonymity.
One of the officials said the list of options included supporting rebel counter-attacks elsewhere with additional weaponry or even air strikes, which "might not reverse the tide of battle, but might cause the Russians to stop and think".
Fight for Aleppo
Recapturing Aleppo would be the biggest victory of the war for regime forces, and a potential turning point in a conflict that until now most outside countries had said would never be won by force.
Aleppo has been divided into regime and opposition sectors for four years, and its opposition zone is now the only major urban area still in the hands of anti-regime fighters supported by the West and Arab states.
The regime laid siege to the opposition zone in July, cutting off those trapped inside from food and medicine.
The last week of bombing has killed hundreds of people and wounded many hundreds more, with no way to bring in medical supplies.
There are only around 30 doctors inside the besieged zone, and eastern Aleppo's two biggest hospitals were knocked out of service by air strikes or shelling on Wednesday.
Bouthaina Shaaban, an adviser to Syrian regime leader Bashar al-Assad, denied regime planes had bombed the hospitals, saying the question was "insulting".
"This is not the first time that such an allegation is uttered and then proven to be absolutely false," she said on Australia's ABC TV.
The multi-sided civil war has killed hundreds of thousands of people, made half the Syrian population homeless, and allowed much of the east of the country to fall into the hands of DAESH who are enemies of all other sides.