Foreign ministers from Russia, Ukraine, Germany and France agreed Wednesday on a series of steps for a resumption of the cease-fire in eastern Ukraine, where fighting between government troops and pro-Russian separatists has taken more than 400 lives since April.

The steps include reopening talks no later than Saturday “with the goal of reaching an unconditional and mutually agreed sustainable cease-fire” to be monitored by the Organization for Security and Cooperation in Europe, their declaration said.

Fighting in eastern Ukraine has increased since a 10-day cease-fire, violated many times, expired Monday. Four Ukrainian troops were killed as government forces carried out more than 100 attacks on rebel positions, a military official said.

Russia supports the proposal to give Ukrainian border guards and OSCE representatives access to Russian territory to take part in controlling two border crossings once the cease-fire resumes, Russian Foreign Minister Sergei Lavrov said after the talks in Berlin.

Border posts have become a key issue, with Ukraine and the United States saying that military equipment and reinforcements are flowing across the border from Russia to the separatists. Moscow denies arming the rebels and describes Russian citizens fighting with them as volunteers.

The ministers’ declaration states that the cease-fire negotiations will be handled by a “contact group,” something Russia has insisted upon. The group, which has held two rounds of peace talks, includes Ukrainians trusted by Russia, the Russian ambassador and rebel leaders.

Since Monday, three troops have died in rebel attacks on government vehicles and checkpoints and 10 have been wounded, a spokesman for Ukraine’s National Security and Defense Council said. The federal border guards said one guard was killed when the Novoazovsk crossing point in the Donetsk region came under attack by rebels.

Donetsk is one of two eastern regions — the other is Luhansk — where separatists have declared independence from the government in Kiev.

Ukrainian officials said pro-Russian rebels had been forced out of three villages. Ukraine said it recaptured a key border post Tuesday at Dovzhanskiy, which rebels had mined with explosives.

A border crossing at Izvaryne was closed Wednesday because of fighting. Ukrainian officials said that rebels shelled troops in the area and that a Ukrainian armored vehicle was destroyed by a mine.

At the small Sjevernyi border crossing to the north, Ukrainian border guards had abandoned their post, leaving three Russian border guards to process the several dozen Ukrainians who passed through Wednesday afternoon.

In the city of Luhansk on Wednesday, a stray missile hit a school near a rebel position, but no injuries were reported.

To the north, a steady, deep rumbling could be heard from shelling near the town of Metalist.