Ukraine's acting president has announced that the ex-Soviet nation's troops have been given orders to withdraw from Crimea following the peninsula's seizure and annexation by Russia.

Acting President Oleksander Turchinov told parliament on Monday that the decision had been taken in the face of "threats to the lives and health of our service personnel" and their families.

His comments came after Russian troops entered a key Ukrainian marine base near Feodosia crowning a gradual take-over of Ukrainian military facilities on the peninsula.

Russia completed its annexation of Crimea last week, following the February ouster of a Russian-leaning government in Kiev.

"The National Defence and Security Council has instructed the Defence Ministry to carry out a re-deployment of military units in Crimea and carry out the evacuation of their families," Turchinov said.

The spokesperson of the Ukrainian forces in Crimea said they had not received orders to withdraw but were aware of the announcement from the president's website.

Ukraine's Defence Ministry confirmed that Russian forces had stormed a Ukrainian military base on the Crimean Peninsula on Monday morning and taken two servicemen captive.

The ministry said when Russian troops seized the marine base in the port of Feodosia on Monday they detained up to 80 Ukrainian servicemen on-site and took two injured Ukrainians away by helicopter.

Russian forces have seized Ukrainian ships and most military bases in Crimea. Over the weekend, Russian forces stormed the Belbek air force base near Sevastopol and detained the commander.

G7 meeting

Also on Monday, US President Barack Obama arrived in the Netherlands to discuss with six other world leaders how to punish Russia for annexing Ukraine's Crimea region, including possibly excluding Moscow from the G8 bloc of rich nations.

Al Jazeera's Robin Forestier-Walker, reporting from The Hague, said that the summit, originally set to discuss nuclear security, was expected to be overshadowed by the events in Ukraine.

"Obama has called for an informal G7 meeting where we think it is highly likely that the Russians will temporarily be kicked out of the G8," our correspondent said.

British Prime Minister David Cameron said leaders from Canada, France, Germany, Italy, Japan and the United States, minus current G8 chairman Russia, must discuss the permanent expulsion of Russia from the group, to which it was admitted in 1998 after it chose a democratic post-Soviet course.