MOSCOW (AFP) ― Germany’s foreign minister said Tuesday he was not optimistic about an end to the conflict in Ukraine because of a “dangerous” escalation of fighting in the east.

Frank-Walter Steinmeier made the downbeat assessment after NATO’s secretary-general spoke of a “very serious buildup” of troops, artillery and air defense systems inside Ukraine and on the Russian side of the border.

Steinmeier was the first senior European minister to visit Moscow since July, in a sign of how relations between the West and Russia have dived to a post-Cold War low over the conflict in eastern Ukraine.

“There are no grounds for optimism in the current situation,” Steinmeier said at a press conference after talks with his Russian counterpart Sergei Lavrov in Moscow.

The German minister warned of a “dangerous situation developing” in Ukraine, which he visited earlier in the day.

He appealed to all sides to stick to the Minsk cease-fire agreement signed on Sept. 5, which appears increasingly threadbare.

Steinmeier said it would be a “great loss” to abandon the frequently violated cease-fire deal, which has stopped most fighting but failed to prevent frequent flare-ups at strategic locations.

“The Minsk agreements are not perfect but it is the only thing that has been supported by all the key players ― the European Union, the United States, the parties to the Ukrainian conflict and Russia,” he said.

Before Tuesday’s meeting got underway, Lavrov said his government hoped “that the ‘point of no return’ has not yet been crossed” in Russia-Europe relations.

But NATO chief Jens Stoltenberg said at an EU defense ministers’ meeting in Brussels that “Russia has a choice.”

“Russia can either be part of a peaceful negotiated solution or Russia can continue on a path of isolation.”

Russia denies any military involvement in the unrest in east Ukraine which has killed over 4,100 people in the past seven months, according to U.N. figures.

Steinmeier, whose country is playing the lead role in mediating the crisis with Russia, flew to Moscow after meeting Ukraine‘s Prime Minister Arseniy Yatsenyuk in Kiev.

Yatsenyuk repeated calls for U.S.-backed negotiations with Russia on “neutral territory.” But Moscow quickly told Kiev that it needed to deal directly with pro-Russian rebels in the region instead.

As the unrest in eastern Ukraine drags on into the ex-Soviet state’s harsh winter, Ukraine’s military said Tuesday that six of its soldiers had been killed in fresh clashes over 24 hours between government forces and rebels.

Russia rejects claims that it provides military backing for the heavily armed separatist rebels in the east.

It also denies that it supplied the anti-aircraft missile which downed Malaysia Airlines Flight 17 in eastern Ukraine in July, killing 298 people, an incident which sharpened the West’s focus on the unrest.

A team overseen by Dutch experts completed a third day of work to recover debris from the wreckage of the doomed Amsterdam to Kuala Lumpur flight and eventually return it to the Netherlands.

Ukrainian security services claimed Tuesday to have thwarted a plot against Dutch Foreign Minister Bert Koenders when he visited the crash site to pay his respects to the victims on Nov. 8.

“We have arrested a very dangerous group of 12 people,” the head of the security service, Valentyn Nalyvaychenko, told a press conference.

“We seized weapons, guns and ammunition. They also had mines and explosives in their possession.”

Russia’s isolation was underlined at the G20 summit in Brisbane at the weekend, when President Vladimir Putin left ahead of schedule after a frosty reception from his fellow leaders.