Russian lawmakers on Tuesday passed a controversial bill allowing authorities to ban foreign NGOs deemed "undesirable" for the state, the latest step in a crackdown condemned by rights activists.

Pro-Kremlin legislators overwhelmingly approved the legislation -- which still has to be voted on by the rubberstamp upper chamber and signed off by President Vladimir Putin -- that targets international non-governmental organizations accused of undermining Russia's "state security."

NATO head Jens Stoltenberg urged Russia on Tuesday for full transparency during snap military exercises in order to avoid potentially catastrophic misunderstandings as the Ukraine crisis stokes tensions.

Transparency is paramount, Stoltenberg said after talks with Russian Foreign Minister Sergei Lavrov on the sidelines of a Council of Europe meeting in Brussels.

Kiev on Tuesday showed off two purported Russian soldiers it captured during a gun battle in the separatist east that Ukraine's pro-Western leadership says proves the Kremlin's direct involvement in the war.

The two wounded men -- recovering in a Kiev military hospital under the guard of masked state security men -- have turned into pawns in a bitter public relations battle being waged by Moscow and Kiev since Saturday's firefight.

U.S. Secretary of State John Kerry took a risky gamble facing derision and much skepticism when he flew to Russia last week for the first time in two years to meet President Vladimir Putin.

But his bet may be beginning to pay off, as two top American diplomats were welcomed in Moscow on Monday for top-level talks for the first time in months on two crises bedeviling global affairs -- Ukraine and Syria.

U.S. Assistant Secretary of State Victoria Nuland called on Monday for concrete progress on a frayed Ukraine peace plan after a flurry of diplomacy with Russia over the crisis rocking the ex-Soviet state. 

Nuland was in Moscow for talks with senior Russian officials that rounded out a week of high-level diplomacy which saw her boss John Kerry jet in to push President Vladimir Putin on implementing the February peace deal. 

Ukraine vowed on Monday to show off two Russian soldiers it claimed to have captured while fighting Moscow-backed forces in the separatist east.

The politically-charged declaration came as a tenuous February truce was broken by more violence that claimed the lives of at least four Ukrainian servicemen.

A Russian tycoon accused of embezzling tens of millions of dollars in a real estate scam was taken into custody in Moscow Sunday after being deported from Cambodia, officials said. 

Sergei Polonsky, who is in his 40s, was detained on an island off the southwestern town of Sihanoukville in Cambodia on Friday for overstaying his visa.

Ukraine has captured two Russian soldiers in its conflict with separatists in the country's east, Kiev said Sunday.

Military spokesman Andriy Lysenko simply named those captured as "Russian soldiers", while giving no further details. 

Russia will not sign the United Nations Arms Trade Treaty that requires governments to ensure their exports will not fuel conflicts, a senior foreign ministry official said Sunday.

"We decided not to join. We weighed all the pros and cons and decided it is not obligatory for us," Mikhail Ulyanov, who heads the foreign ministry's non-proliferation and arms control department, told TASS state news agency.

Ukrainian President Petro Poroshenko on Friday enacted laws banning Soviet symbols and communist-era propaganda, his office said, after the measures drew strong Russian criticism when they were approved by parliament last month.

The measures come as Ukraine's pro-Western government seeks a complete break with its Soviet past and as its soldiers fight rebels in its east allegedly supported by Russia.