German Chancellor Angela Merkel said that she could not rule out new sanctions against Russia over the Ukraine crisis, following talks with President Petro Poroshenko in Kiev.
She said: "We cannot rule out thinking about further sanctions if things do not progress,"
Ms Merkel called for a "bilateral ceasefire and effective border controls" to help end the conflict in the east of the country.
Earlier, lorries from Moscow's controversial aid convoy to rebel-held parts of Ukraine returned to Russia .
The acting head of the mission observing the Russian border post known as Donetsk said that some of the vehicles had begun passing through but could not provide a number.
Russian news agencies reported that several dozen had arrived at the border.
The United States demanded that Moscow remove an aid convoy it sent into rebel-held eastern Ukraine without permission.
It accused Russia of a flagrant violation of the sovereignty of its former Soviet neighbour and threatening more sanctions.
Moscow, which has thousands of troops close to the Russian side of the border, warned against any attempt to "disrupt" the convoy.
It said it was purely humanitarian.
It did not say what action it might take if the Ukrainian military intervened.
NATO's top military commander said the movement of trucks looked like a disguised attempt to reinforce separatist forces.
The Western defence alliance said Russian troops had been firing artillery across the border.
Within Ukraine there has been a major escalation of military support for pro-Moscow rebels since mid-August.
Moscow denies backing the rebels militarily but the United States and European Union have imposed sanctions and the Kremlin has retaliated, renewing some of the chill of the Cold War.
NATO has deployed extra troops in member states bordering Russia,including former Soviet Baltic states and ex-communist Poland.
Ukrainian President Petro Poroshenko described the entry of the trucks without Kiev's permission as a "flagrant violation of international law".
A senior security chief said Ukrainian forces would let them pass to avoid "provocations".
Kiev called on international allies to unite in "a decisive condemnation of illegal and aggressive actions" by Russia.