Pro-Russian rebels in eastern Ukraine called a cease-fire Monday in fighting against government forces, matching a truce announcement made three days earlier by President Petro Poroshenko.

The announcement by the separatists opens the way for an easing of the crisis that has left hundreds dead in the former Soviet republic and led to the worst standoff between Russia and the United States and its European allies since the Cold War.

The prime minister of the self-proclaimed Donetsk People’s Republic, Alexander Borodai, said separatist fighters in the Donetsk region would halt hostilities until June 27 as long as government troops do the same. In comments broadcast on Rossiya 24 television, Borodai said his group is seeking talks with the Ukrainian government before that date, when Poroshenko’s cease-fire is also due to expire.

Rebels in the other breakaway, mainly Russian-speaking region, Luhansk, also agreed to a truce, Rossiya 24 reported. The announcement came after talks in Donetsk that involved former Ukrainian President Leonid Kuchma and Russia’s ambassador to Ukraine, Mikhail Zurabov.

The United States and the European Union have accused Russian President Vladimir Putin, who annexed Ukraine’s Crimea peninsula in March, of allowing weapons to flow to the separatists across the Ukrainian border and building up its military forces along the frontier again.

“At least a few thousand more troops” from Russia have been sent to the border, a U.S. Defense Department spokesman, Army Col. Steve Warren, told reporters at the Pentagon. “Russia has accumulated artillery at a deployment site in southwest Russia and we believe Russia might soon provide this equipment to separatist fighters.”

U.S. President Barack Obama spoke with Russian President Vladimir Putin to urge him to use his influence to promote peace and stability in Ukraine, White House spokesman Josh Earnest told reporters.

Putin is due to visit Vienna Tuesday, where he will hold talks with Austrian President Heinz Fischer.

Poroshenko suggested that Russia send inspectors to join a monitoring team from the n Organization for Security and Cooperation in Europe to witness the cease-fire by government forces, according to a statement published on his website.

The developments in eastern Ukraine came after EU foreign ministers demanded concrete action from Putin to end the violence.

“The EU calls on the Russian Federation to support the peace plan and to adopt effective measures to stop the continued flow of illegal fighters, arms and equipment,” the ministers said in a statement in Luxembourg. They urged Russia “to use its influence on the separatists to stop the violence and lay down their arms, to continue withdrawing and refrain from gathering troops again near the Ukrainian border.”

Putin put more than 65,000 troops on combat alert over the weekend and called a snap military drill in the Central Military District, which doesn’t share a border with Ukraine, involving 5,500 pieces of equipment. It was the biggest exercise since the annexation of Crimea.

The United States expanded sanctions last week to more people linked to the insurgency. U.S. and European officials have warned that more painful penalties affecting Russia’s access to financial markets, technology and military hardware may come as early as this week if Putin fails to curb tensions.