(REUTERS/Brian Snyder)
US Secretary of State John Kerry riding a boat through the Mekong River Delta in 2013. It was the first time Kerry had returned to the Mekong River Delta since he commanded a swift patrol boat during the Vietnam War.

The new deadline for Syrian President Bashar Assad's transition out of power: August 1, US Secretary of State John Kerry said in a news conference at the State Department on Tuesday.

"The target date for the transition is the first of August," Kerry told reporters. "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."

The ultimatum was reminiscent of Kerry's warning in 2011 that Assad's days were "numbered," as well as President Barack Obama's "red line" speech in 2012 outlining the conditions — namely, the use of chemical weapons — that would prompt the US to take action against the embattled president. But then, as now, Kerry did not specify which "track" Washington would take to force Assad's ouster.

"That is for the future," Kerry said.

Kerry responded to questions about Assad's departure after announcing a new plan to end the latest wave of violence in Syria's largest city, Aleppo, where more than 250 civilians have been killed in less than a week by government airstrikes and rebel shelling.

Under a new cease-fire arrangement, US and Russian military officials "will be sitting at the same table" at a coordination center in Geneva to monitor and document any violations of the truce, Russian Foreign Minister Sergei Lavrov said Tuesday in a news conference from Moscow with the UN's envoy to Syria, Staffan de Mistura.

The truce will include Aleppo moving forward.

(Abdalrhman Ismail/Reuters)
Civil defense members resting after an airstrike on the rebel-held Tariq al-Bab neighborhood of Aleppo, Syria, on April 23.

Russia — a staunch ally of Assad — had initially refused to include Aleppo in the cessation of hostilities agreement because of Al Qaeda affiliate Jabhat al-Nusra's presence in parts of the city. It used the group to justify the continued airstrikes.

As a result, joint US-Russian efforts to end regime bombardments there have largely been made on Moscow's terms, and Kerry acknowledged that negotiators were still trying to figure out how to target Nusra — which is not party to the cessation of hostilities agreement — without hitting rebel groups who have agreed to abide by the cease-fire.

"Are they somehow commingled? Are they fair game? These are the kinds of things that have to be worked out so that there's no misunderstanding" about "who is doing what, where, when, and how," Kerry said. "We don't control the terrorists."