The UN Security Council on Friday held a brief "private meeting" followed by consultations to discuss concerns about rapidly changing events in Ukraine, AFP reports.
The meeting was held at the request of Lithuania, which holds the council's rotating presidency, officials said.
Ukrainian ambassador Yuriy Sergeyev said Russia's expanding military presence in Crimea was a "challenge to the security and peace in the region and challenging the territorial integrity."
Addressing reporters after briefing the Security Council, he said Ukraine considered the Russian troops aggressive and a move towards encouraging "separatism" and "clashes."
In Kiev, a Ukrainian official said 13 Russian aircraft carrying nearly 2,000 suspected troops have landed at a military air base near the regional capital of the restive Crimean peninsula.
Sergeyev said he had been informed about 10 aircraft and described them as military attack MI-24 helicopters.
Meanwhile, Ukraine's ousted president Viktor Yanukovych insisted he had not been overthrown, surfacing in Russia on Friday after a week in hiding and hitting out at the new pro-Western powers in Kiev.
Speaking to reporters en route to Friday's meeting, British ambassador Mark Lyall Grant called on all sides to "de-escalate the situation and reduce tensions."
"We think it's an important opportunity to set out our concerns about developments, particularly in the Crimea," he said.
Britain would also "underline the importance that we hope all member states attach to the unity and territorial integrity of Ukraine," he added.
Asked what he expected from the talks, Russian ambassador Vitaly Churkin shot back: "I have no idea" as he rushed into the talks.
Envoys from the 15 council member states met behind closed doors for the briefing from Ukraine's ambassador and UN undersecretary general for political affairs Jeffrey Feltman.
It was not clear if the council would issue a statement following the meetings.
Robert Serry, a special envoy of UN Secretary General Ban Ki-moon, has been in Kiev for the past several days to deliver Ban's "message about the importance of the sovereignty of Ukraine and its territorial integrity," a UN spokesman said.
Serry would also emphasize "the need to reduce tensions and to ensure that the process under way is inclusive, peaceful and conducive to dialogue," said spokesman Martin Nesirky.
When asked about the situation in Crimea, Nesirky said the UN was "obviously concerned," calling on all sides to reduce tensions.
Ukraine's pro-EU interim authorities said they had regained control of two Crimean airports seized during what they branded an "armed invasion" by Russian troops.