US Secretary of State John Kerry gestures as he speaks with Russian Foreign Minister Sergei Lavrov during their meeting in Moscow. (Sergei Chirikov/EPA)

Secretary of State John F. Kerry called Thursday for unity between Russia and the United States to help end the conflict in Syria, saying this week’s terrorist attacks in Brussels have “underscored the urgency” of defeating the Islamic State.

In brief remarks at the Russian Foreign Ministry’s guest house in downtown Moscow, Kerry and Foreign Minister Sergei Lavrov praised a recent cease-fire agreement in Syria that seeks to jumpstart U.N.-brokered peace efforts in Geneva.

After talks with Lavrov, Kerry is slated to meet with President Vladimir Putin at the Kremlin, where Kerry will seek to gauge Moscow's readiness for a political transition in Syria that could remove President Bashar al-Assad from power.

[Kerry pledges U.S. help in Brussels attack probes]

Russia launched military airstrikes in the five-year old Syria conflict last year, bolstering its ally Assad and sending the armed opposition reeling, including factions backed by the West and its partners.

Putin, who drew down his forces in Syria by about half last week, has maintained that Assad is an important bulwark against terrorism in Syria and warned that Moscow could rebuild its forces in Syria within hours.

Kerry said that the bilateral talks in Moscow were a chance to "prove that two powerful nations that have been able to find cooperation in the past few years despite differences have the ability in the face of those differences to do what is necessary to meet the challenge."

[Syria shows that Russia built an effective military. Now how will Putin use it?]

Kerry noted that a recent cease-fire agreement had led to a “fragile but beneficial reduction in violence” in the Syrian conflict, which has sent hundreds of thousands of migrants fleeing the violence into Europe. He cited estimates saying the reduction in violence was as high as 90 percent.

Lavrov, 66, echoed Kerry’s statements, saying that cooperation between Moscow and Washington had “achieved success because we were working on the basis of equality.” He also said that the talks on Thursday would address the crisis in southeast Ukraine, which has seen a considerable reduction in violence but little progress toward a political solution since peace accords were signed in Minsk more than a year ago.

The United States imposed sanctions following Russia’s annexation of Crimea from Ukraine in 2014, saying that decision had isolated Putin from the world community. Ukraine has been locked in battles between pro-Russian rebels and forces of the Western-allied government in Kiev.

But Russia’s entrance into the conflict in Syria and growing concerns about terrorism in Europe, especially following the Paris attacks last November, have re-invigorated contacts between Moscow and Washington.

[Kerry’s desperate push on Syria]

Moscow wants those talks to be closer, which it says is necessary to fight international terrorism, but will also prove that Russia has escaped international isolation over the Ukraine conflict.

Last week, Moscow threatened it would begin using force to unilaterally enforce the Syria ceasefire if it could not hammer out a more explicit rules with the United States military to enforce the cease-fire.

Col. Steve Warren, a spokesman for the U.S.-led coalition in Iraq and Syria, said in response that there was no delay and that the issues "have been discussed at length already, and continue to be discussed, in a constructive manner."

Kerry is also expected on Thursday to urge Putin to release Lt. Nadiya Savchenko, who was recently sentenced by a Russian court to 22 years in prison for directing mortar fire that killed two Russian journalists in southeast Ukraine in 2014.

Savchenko's case has become a cause celebre in Kiev, where she was elected to parliament and conferred the title of Hero of Ukraine while waiting for trial in custody in Russia. Ukrainian President Petro Poroshenko called the conviction a "show trial." Ukrainian officials said they were ready to trade several alleged Russian servicemen arrested in southeast Ukraine in exchange for Savchenko's freedom.

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