Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

Ukraine is preparing to withdraw its soldiers and their families from Crimea, officials in Kiev announced Wednesday.

Speaking at a press conference, Andriy Parubiy, Secretary of the National Security and Defense Council Ukraine, said that “we are working on a plan to quickly and effectively transport not only military servicemen but also their families to the territory of mainland Ukraine.”

Parubiy said that the ministry of foreign affairs has asked the United Nations to classify Crimea as a demilitarized zone and also called for the withdrawal of Russian troops.

He also said Ukraine will hold military maneuvers with the countries that signed the 1994 Budapest Memorandum. He didn't elaborate.

The document was signed by the U.S., Britain and Russia to guarantee Ukraine's territorial integrity when it surrendered its share of Soviet nuclear arsenals to Russia after the Soviet Union broke up in 1991. Ukraine has accused Russia of breaching the agreement by taking over the Crimean Peninsula.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

U.S defense officials were caught off guard by the announcement but quickly pointed out that U.S. and NATO forces have conducted an annual military exercise with Ukrainian forces each July for several years.

Ukraine has been powerless to prevent Russian troops from taking control of Crimea, which Russian President Vladimir Putin annexed on Tuesday.

A day later, masked Russian-speaking troops moved into Ukraine's naval headquarters in the Crimean city of Sevastopol, detaining the head of Ukraine's navy and seizing the facility. They faced no resistance.

Earlier on Wednesday, Acting President of Ukraine Oleksander Turchinov gave the "self-proclaimed government of Crimea" a deadline 3 p.m. ET to release all hostages and stop provocations on the peninsula, according to Turchinov's website.

Meanwhile, senior military officials tell NBC News that “thousands” of Russian troops staged along the Ukrainian border have raised serious concerns at the Pentagon. According to one senior official, while there’s no indication the troops are planning to enter Ukraine, they’re so close they could easily mount an invasion without warning.

“It’s like they’re on a hair trigger,” the official said.

From Russia, Crimea is accessible only by air or sea, so U.S. officials speculate that at some point the Russians may decide to seize a slice of Eastern Ukraine by military force to provide a “land bridge” from Russia to Crimea,

Officials stress that there’s been no discernible change in the number of forces or equipment over the past several days, but their sheer presence has everyone’s attention.

— Hasani Gittens, Peter Jeary and Jim Miklaszewski, with The Associated Press