FEODOSIA, Crimea - As former comrades saluted them from outside a base overrun by Russian forces, Ukrainian marines in Crimea piled into buses Tuesday to head back to the mainland.

It was a low-key exit from this eastern Black Sea port, with fewer than a dozen friends and relatives on hand to bid the marines farewell. A troop transporter bearing black Russian military plates trailed the bus as it pulled away.

Their departure came as Ukraine's defense minister stepped down after harsh criticism for authorities' often-hesitant reaction to Russia's annexation of Crimea, which was formalized following a hastily organized referendum this month. And while Ukraine struggled to deal with its humbling by Russia, it also faced the menace of seething Ukrainian nationalists angered by the police killing of a leading radical.

Troops were given the stark choice of either staying in Crimea and switching allegiance to serve under Russia's military, or leaving the peninsula to keep their jobs with the Ukrainian defense forces.

"The Russians threatened, intimidated, bullied and tried to get us to switch sides to Russia. It has been very difficult to resist this enormous pressure but I have made a choice that I can live with," Senior Lt. Anatoly Mozgovoy told The Associated Press after arriving in the Ukrainian city of Genichesk .

"We were greeted as heroes in Ukraine. I was able to breathe freely for the first time in months," the 30-year-old Mozgovoy said.

He said he left behind his wife and 7-month-old daughter, who were staying with his mother-in-law in Crimea until he finds out where he is being permanently deployed.

So far, 131 Ukrainian marines have left Crimea, the defense ministry said. They were being temporarily stationed at a military barracks in Genichesk but their final destination was still unclear.

At a summit on nuclear security in The Hague, Netherlands, President Barack Obama said Russian troops would not be dislodged from Crimea by force.

He noted that one of the achievements of his first nuclear summit in 2010 "was Ukraine's decision to remove all of its highly enriched uranium from its nuclear fuel sites."

"Had that not happened, those dangerous nuclear materials would still be there now. And the difficult situation we're dealing with in Ukraine today would involve yet another level of concern," Obama said.

In an address to parliament in the capital, Kiev, Defense Minister Igor Tenyukh denied that he had failed to issue clear instructions to his troops but reserved the right to resign. The order to withdraw from Crimea was issued Monday, a week after many bases had already been stormed and seized by pro-Russian forces.

Lawmakers initially refused Tenyukh's resignation but later accepted it and replaced him with Col. Gen. Mykhailo Koval.

About 4,300 Ukrainian servicemen and 2,200 of their relatives have asked to leave Crimea, Tenyukh said Tuesday. That means about two-thirds of the 18,800 military personnel and relatives that he said were stationed on the Black Sea peninsula were taking their chances in Crimea.

Ukrainian marines prepare to leave their base in Feodosia, Crimea, on Tuesday, March 25, 2014. Lawmakers in Ukraine accepted the resignation of the defense minister Tuesday as thousands of troops began withdrawing from the Crimean Peninsula, now controlled by Russia.

Tenyukh said accommodations for incoming soldiers were being prepared at boarding houses and other facilities in Kiev. Oleksandr Rozmaznin, deputy chief of operations for Ukraine's armed forces, said navy troops were being redeployed in port cities along Ukraine's southern mainland - in Odessa, Mykolaiv and Kherson.

The Defense Ministry, meanwhile, said 11 of its servicemen have been abducted by Russian troops and remain unaccounted for, including Col. Yuliy Mamchur, a commander who earned wide acclaim in Ukraine for defying besieging pro-Russian forces until his base was stormed over the weekend.

Ukraine's new government is struggling to consolidate control amid ominous signals of discontent from Right Sector, a radical nationalist movement that played a key role in the anti-government demonstrations that prompted President Viktor Yanukovych to flee to Russia in February.

One radical, Oleksandr Muzychko, was shot dead overnight as he was being detained by police, the Interior Ministry said Tuesday.

Moscow has cited the alleged influence of nationalist groups like Right Sector to justify its hasty annexation of Crimea, which has a large Russian majority.

Russian state television, which is widely viewed by Ukraine's Russian-speaking population in the east, has regularly aired lurid reports on Muzychko's antics as part of what media analysts say is a sustained effort to undermine the government in Kiev.

But many in Ukraine downplay the group's importance and it has no posts in the new government. Police say Muzychko was being sought for organized crime links, hooliganism and threatening public officials.

Right Sector leader Dmytro Yarosh lashed out at his killing.

"We cannot silently watch as the Interior Ministry carries out active anti-revolutionary activities," Yarosh said.

His group demanded the immediate resignation of Interior Minister Arsen Avakov and the arrest of the head of the Sokol special forces.

Amid the country's political turmoil, Ukraine's economy is in a dire state and representatives from the International Monetary Fund have been holding talks with the new government for weeks on the terms of a potential bailout.

Officials in Moscow, meanwhile, warned Kiev that the country's new government will have to pay more for Russian gas. President Vladimir Putin's spokesman, Dmitry Peskov, said a gas discount that Russia had previously given Ukraine was linked to the Russian Black Sea fleet's lease in Crimea and is no longer valid.

But he added that the Russian natural gas giant Gazprom would have to set the new price.

In November, Russia agreed to help prop up Yanukovych's teetering government by selling Ukraine gas at $268.50 per thousand cubic meters, but that discounted price has been scrapped. Ukraine's Energy Minister Yury Prodan said Tuesday that Kiev would pay Gazprom no more than $387 per thousand cubic meters for gas.

The U.S. and the EU have both hit Russia with sanctions for annexing Crimea, and NATO member Norway on Tuesday suspended joint activities with Russia's military. But Russia has so far shrugged off the sanctions, including being tossed out of the elite Group of Eight developed nations.

Officials say the other G-8 nations will meet, without Russia, in Brussels in June.