Secretary of State John Kerry warned Syria and its allies in Moscow and Tehran Tuesday that they have until August for starting a political transition to move President Bashar Assad out, or they risk consequences of a new U.S. approach toward ending the five-year conflict.

Kerry failed to specify what type of consequences the Syrian government and its backers face if Assad isn’t ousted. It’s unlikely the White House would approve a more active American combat role in the country. The administration is more likely to approve giving Saudi Arabia and the moderate rebels new weapons to fight Assad.

"The target date for the transition is 1st of August," Kerry told reporters at the State Department. "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 top American diplomat spoke following a meeting between the U.N. envoy for Syria and Russia's foreign minister in Moscow on Tuesday, a day after discussions with Kerry in Geneva. The goal was to restore a partial truce that has all but unraveled amid 12 straight days of bitter fighting in Aleppo, Syria's largest city.

Kerry condemned last week’s hospital attack in the city that killed at least 20 people. He said the U.S. rejects violence against civilians, whether it's by Assad's government or Western-backed opposition groups.

But Kerry saved his sharpest comments for Assad and his government's two key military, economic and diplomatic lifelines: Russia and Iran.

"If Assad does not adhere to this, there will clearly be repercussions," Kerry warned. "One of them may be the total destruction of the cease-fire and then go back to war. 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."

Kerry admitted the U.S. and Russia were working on the details of a stronger ceasefire that would include Aleppo and prevent the metropolis from failing. He said leaders on all sides must strive for peace.

On its face, the threat of continued fighting doesn't seem to carry much weight. Assad has aggressively sought to crush any and all opposition groups in a war that emerged from the government's violent repression of largely peaceful, Arab Spring-inspired protests in 2011. Despite a death toll that numbers in the hundreds of thousands, Russian planes and Iranian troops continue to fight alongside the Syrian military.

Kerry also appeared to undermine his own selling of a truce by stressing that the opposition would never accommodate Assad's leadership. The current U.N.-endorsed transition plan for Syria says nothing about Assad relinquishing power or being prevented from running for an eventual re-election as president. His family has ruled Syria for four decades.

"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," Kerry said.

"As long as Assad is there, the opposition is not going to stop fighting," he said.

Kerry said he has told his counterparts in Moscow and Tehran that calm won't prevail in Syria if they're not prepared to move quickly toward a new Syrian government.

"Assad cannot reunite the country — it's that simple," Kerry said.

"Having gassed his people, barrel bombed his people, dropped bombs on hospitals, driven 12 million people out of their homes, tortured people, starved people, what kind of legitimacy should somebody who's committed these kinds of atrocities suddenly claim to run the country? It's pretty hard for anybody to understand how you make peace out of that record of chaos and depravity."

The Associated Press contributed to this report.