MOSCOW — After lengthy meetings with both Russian President Vladimir Putin and the country’s foreign minister, Secretary of State John Kerry said Thursday that Washington and Moscow have reached an understanding on how the 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 United States 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 United States 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.’’

US 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.

The main Syrian opposition group has wrapped up the latest round of indirect peace talks by urging Russia to ‘‘use its leverage’’ on Assad’s government to fulfill international hopes for a political transition.

In Geneva, Bassma Kodmani, a leader of the opposition High Negotiations Committee, told reporters Thursday that it wants greater access for humanitarian aid and decried continued sieges by government forces on Syrian municipalities.

The United States and Russia have been at odds over Syria since the conflict began more than five years ago, with Washington demanding Assad’s ouster and Moscow saying it is up to the Syrian people to determine their leadership.

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.

The other current significant difference between the United States and Russia is the situation in Ukraine. Washington accuses Moscow of not doing enough to push pro-Russian separatists in the east to comply with a cease-fire. Russia, meanwhile, has complained that the Ukrainian government is dragging its feet on implementing the cease-fire.

Fighting in Ukraine’s industrial heartland, which has close ties to Russia, has killed more than 9,100 people and left large swaths of land under rebel control. Germany, France, and Russia mediated talks between the Ukrainian government and the Russian-backed separatists in Minsk, Belarus, which resulted in the truce agreement.

That has largely held, but none of the political elements, including calling a local election, has been implemented.

Kiev insists it can’t hold the vote because it cannot guarantee security for election officials. For their part, the rebels have said they won’t allow Ukrainian right-wing parties to run, which the Ukrainian government says also makes the election impossible.

Kerry was to raise concerns about a recent sharp increase in cease-fire violations and press Russia to do more to get the separatists in line. Unless there is ‘‘true quiet’’ and full access for cease-fire monitors, US officials say it will be difficult to get progress on other parts of the Minsk deal.