Germany's foreign minister shuttled between Kiev and Moscow on Tuesday in an effort to avert all-out war between Ukrainian government forces and Russia-backed separatists.
NATO's new chief, meanwhile, warned of a "serious military buildup" by Russia along Ukraine's eastern border that is escalating tensions and the risk of renewed fighting.
Presidents Vladimir Putin of Russia and Petro Poroshenko of Ukraine signed a cease-fire agreement in the Belarus capital of Minsk on Sept. 5 in which they agreed to halt armed clashes in separatist-occupied eastern Ukraine.
The truce tamped down fighting in most areas for a few weeks, but elections on both sides of Ukraine's pro-Russia and pro-Europe divide over the last month have empowered new political forces that are on a collision course in the 7-month-old conflict.
A majority of Ukraine's Supreme Council deputies are committed to recovering Russian-annexed Crimea and the separatist-controlled Donetsk and Luhansk regions, while Putin's proxies in eastern Ukraine say they will fight to the end to keep the territory they have seized and proclaimed independent from Kiev's rule.
At two summits in Asia last week, Putin faced sharp criticism for his aggressive moves in Ukraine. German Chancellor Angela Merkel sounded an alarm Monday after the Group of 20 meeting in Brisbane, Australia, demanding that forced border changes like Moscow's seizure of Crimea "must not become accepted" and warning that a long conflict between Russia and Europe may lie ahead.
Merkel then dispatched German Foreign Minister Frank-Walter Steinmeier to the Ukrainian and Russian capitals on a mission to assess "the chances of avoiding a new spiral of violence in eastern Ukraine," he told reporters before embarking on the diplomatic shuttle on Tuesday.
Steinmeier first arrived in Kiev for talks with Poroshenko and Ukrainian Prime Minister Arseny Yatsenyuk, then headed to Moscow for meetings with Kremlin leaders -- the first visit by a European foreign minister since July to Putin's sanctioned and isolated regime.
Russian Foreign Minister Sergei Lavrov signaled Moscow's low expectations of the diplomatic foray in saying before Steinmeier arrived that he hoped the "point of no return has not yet been crossed" in Russia's relations with Europe.
"No one expects a breakthrough" in the Ukraine crisis from Steinmeier's visit, Lavrov said, adding that Moscow nonetheless "values regular dialogue with Berlin."
During talks between the two foreign ministers, Steinmeier sought to encourage the Kremlin to use its influence with the separatists in Ukraine to end the crisis that has taken more than 4,100 lives since April.
"That is why I am here, to voice our expectations, our confidence that Russia will help in overcoming the current worsening of the situation" in Ukraine, Steinmeier told Lavrov, the Tass news agency reported. He also impressed upon his Russian counterpart that there is no military solution to the Ukraine crisis, Tass said.
As the German diplomat urged restraint, NATO's new political chief reiterated warnings issued by its top commander last week that Russian troops and armor have entered eastern Ukraine and taken up strong positions along its border.
"This is a serious military buildup and we call on Russia to pull back its troops," NATO Secretary-General Jens Stoltenberg said in Brussels.
"Russia has a choice," the NATO chief said. "Russia can either be part of a peaceful negotiated solution or Russia can continue on a path of isolation. The international community calls on Russia to be part of the solution."
Pro-Russia separatists in eastern Ukraine occupied government buildings in March and April, inspired by Moscow's invasion of Crimea and swift annexation of the peninsula that is home to Russia's Black Sea fleet and other vital military installations. Crimea was invaded and seized days after Ukraine's Kremlin-allied president, Viktor Yanukovich, was ousted Feb. 21 by a rebellion sparked by his refusal to sign an association agreement with the European Union -- a move opposed by Moscow and some Russian-speaking residents of eastern Ukraine.