Moscow — After lengthy meetings with both Russian President Vladimir Putin and the country’s foreign minister, U.S. Secretary of State John Kerry said Thursday that Washington and Moscow have reached an understanding on how the fragile cease-fire in Syria can be strengthened.

At a midnight news conference with Foreign Minister Sergey Lavrov, Kerry said this week’s terrorist attacks in Belgium bolstered determination to bring stability to Syria. The Islamic State group, which claimed responsibility for the Brussels attacks, holds substantial territory in Syria.

The attacks “are a stark reminder — they are a violent reminder of what we are trying to do,” Kerry said.

Lavrov said, “We have agreed to continue coordinating activity toward fixing the cessation of hostilities regime.”

Kerry said the U.S. and Russia will press for expanded humanitarian access in Syria and agreed that the Syrian regime and its opponents need to begin releasing detainees as soon as possible.

They agreed on a schedule for drafting a political solution and said the next step in the Geneva talks on Syria is to firm up how a political transition would work.

At the start of his meeting with Putin, which lasted more than four hours, Kerry hailed a cease-fire in Syria brokered by the U.S. and Russia, saying it had allowed Syrians “to taste and smell the possibilities of what it means to have a huge reduction in violence and to receive humanitarian assistance.”

U.S. officials “obviously also have some ideas about this and how we can most effectively make progress in Geneva and begin the very serious and difficult work of the transition,” Kerry said, referring to Syria peace talks in Geneva.

Kerry’s meetings were arranged after Putin made a surprise announcement last week that Russian troops would partially withdraw from Syria after five months of military operations in support of Assad’s government.