US Secretary of State John Kerry has warned Syrian President Bashar al-Assad of the consequences of a new US approach if he does not accept a political transition in the next few months.

“The target date for the transition is 1st of August,” Kerry told reporters at the State Department on Tuesday.

“So we're now coming up to May. So either something happens in these next few months, or they are asking for a very different track,” he added.

The warning came amid a surge in violence in the northern city of Aleppo, putting further strain on a February 27 ceasefire brokered by Russia and the United States.

Kerry said he still hoped negotiations could restore the truce deal to include Aleppo, but warned that there would be repercussions if the Syrian government violated the agreement.

A Syrian man walks past destroyed buildings on May 2, 2016, in Aleppo. (AFP photo)

“If Assad does not adhere to this, there will clearly be repercussions, and one of them may be the total destruction of the ceasefire and then go back to war,” he said.

“I don't think Russia wants that. I don't think Assad is going to benefit from that. There may be even other repercussions being discussed. That is for the future.”

The top US diplomat made the comments following a meeting between the UN special envoy for Syria and Russia's foreign minister in Moscow on Tuesday, a day after emergency discussions with Kerry in Geneva.

Kerry (L) looks at his watch next to UN Syria envoy Staffan de Mistura during a press briefing following a meeting on May 2, 2016 in Geneva. (AFP photo)

Kerry said that Washington and Moscow were in the process of working out the details of a more durable ceasefire in Aleppo to prevent the flashpoint city from falling.

“Everybody at the table has said you can't end this as long as Assad continues, because Assad cannot reunite the country. It's that simple,” Kerry said.

Russian Foreign Minister Sergei Lavrov said a new unilateral ceasefire by the Syrian government could be announced within “the next few hours” for Aleppo. The UN Security Council is set to convene Wednesday to discuss the crisis in the city.

Sergei Lavrov (R) meets with de Mistura in Moscow on May 3, 2016. (AFP photo)

Kerry reiterated that the United States would never accept a transition that included President Assad.

“If Assad's strategy is to somehow think he's going to just carve out Aleppo and carve out a section of the country, I got news for you and for him - this war doesn't end,” he said.

On Tuesday, 19 people were killed by militant rocket fire in government-controlled areas of Aleppo, according to the so-called Syrian Observatory for Human Rights. State media said a rocket attack on a hospital killed at least three people and wounded 17 others.

US officials have frequently warned of consequences if Assad refused to step down as part of a broader peace agreement to end the five-year conflict.

Syria has been gripped by a foreign-backed militancy since March 2011. UN special envoy Staffan de Mistura has estimated that over 400,000 people have been killed in the conflict.

Damascus accuses the US and some of its regional allies, in particular Saudi Arabia, Qatar and Turkey, of conducting a proxy war against Syria.