Moscow - Russia is tightening its military grip on Crimea despite appeals from Ukraine and the international community, which is urging Russia to withdraw its troops from the region.

Russia is reportedly increasing its military build-up on the Black Sea peninsula and tightening its control despite calls from Western powers to stop. The Russian Army gained control of 11 border guard units on Saturday and one on Sunday.

NATO Secretary General Anders Fogh Rasmussen expressed strong support for Ukraine and renewed his call on Russia to withdraw its troops from the Crimea region.

In an interview with Germany's largest mass-circulation daily, Bild, Rasmussen said, "In the 21st century, no one should attempt to draw new borders on the map of Europe."

After the Crimean parliament voiced its support for joining Russia, the region is set to hold a referendum on the issue early next week. The U.S. warned it would not recognize the referendum.

Rasmussen described the current crisis as "the greatest threat to European security since the end of the Cold War" and criticized Russia for violating the territorial integrity and sovereignty of Ukraine. "This is against international law and international obligations of Russia," he said.

Rasmussen said the U.S. deployed additional fighter jets in a bid to augment air defense capabilities of the Baltic countries, which he said showed the "seriousness of the Alliance."

He added, "We call on Russia to respect its international obligations and stop the military escalation in Crimea. We call on Russia to withdraw its troops and also not to interfere in other regions of Ukraine."

Rasmussen also praised Ukrainian leaders for their vigilance, stressing that the Ukrainian military has held back "despite enormous pressure."

Meanwhile, ousted Ukrainian President Viktor Yanukovych will hold a press conference in the Russian city of Rostov on Tuesday, according to the Russian news agency Itar-Tass.

Turkey's Prime Minister Recep Tayyip Erdoğan spoke with German Chancellor Angela Merkel on the phone about Ukraine. According to the statement from the prime minister's office, Erdoğan and Merkel discussed bilateral affairs and recent incidents in Ukraine.

"Both leaders agreed that the recent incidents in Ukraine are extremely worrisome. The sovereignty, territorial integrity and political unity of Ukraine should absolutely be protected. The possibility of skirmish in Crimea should be eliminated. The referendum envisaged to be conducted on March 16 is both extremely unfavorable and against the law," the statement read.

Erdoğan and Merkel stressed that the observance of both international law and bilateral-multilateral agreements is of the utmost importance.

"The two leaders confirmed that efforts for establishment of an International Contact Group and Commission of Investigation are of capital importance," the statement continued. "The leaders arrived at a consensus about the requirement that the U.N. Security Council, the OSCE [the Organization for Security and Co-operation in Europe] Council of Ministers and Committee of Ministers of the Council of Europe should deal with the matter as soon as possible."

Erdoğan said he is ready to contribute to the International Contact Group because of Turkey's close relations with Ukraine and Russia and bonds with Crimean Tatars.

U.S. President Barack Obama called six world leaders, including British Prime Minister David Cameron, French President Francois Hollande and Italian Prime Minister Matteo Renzi, about Ukraine on Saturday to discuss the ongoing crisis, the White House said.

As the Obama administration is struggling to find a way of forcing Russia to remove its troops from Crimea, former Secretary of Defense Robert Gates said, "It is already too late to prevent the contested region from being absorbed into Russia. I do not think that Crimea will slip out of Russia's hand." When pressed for clarification, Gates replied he thought Crimea was "gone."

European Union leaders and the United States repeatedly denounced the referendum as contrary to the Ukrainian constitution and illegal.

European leaders ramped up the pressure on Russia to engage in bilateral talks with Ukraine in an effort to de-escalate military tensions between the two countries.