* Ukrainian PM says Crimea is and will remain part of Ukraine
* Yatseniuk says Ukrainian military will respond if Russia escalates
* Prime minister calls Crimea referendum decision "illegitimate" (Adds details, quotes from prime minister's press conference)
By Jan Strupczewski and Luke Baker
BRUSSELS, March 6 (Reuters) - Ukraine is ready to use military force to defend itself if Russia further expands its presence on Ukrainian soil, Prime Minister Arseny Yatseniuk said on Thursday, emphasising that Ukraine would not be made subordinate to its neighbour.
Speaking at a news conference after meeting European Union leaders in Brussels, Yatseniuk said Crimea was and would remain an intregal part of Ukraine and dismissed a decision by Crimea's parliament to join up with Russia.
"In case of further escalation and military intervention into Ukraininan territory by foreign forces, the Ukrainian government and military will act in accordance with the constitution and laws," Yatseniuk said.
"We are ready to protect our country."
Ukrainian forces have so far not responded to the takeover of the Crimean peninsula by Russian forces, although there have been tense exchanges caught on film between Ukrainian soldiers and armed Russians in unmarked uniforms in some areas.
The parliament in Crimea voted unanimously earlier on Thursday to leave Ukraine and join Russia, while maps on at least one Russian TV station started showing the peninsula as being part of Russian territory, not of Ukraine.
Yatseniuk called the parliament's decision illegitimate and said a referendum on Crimea's status, called by the region's vice premier for March 16, had no legal grounds.
"Crimea is, was and will be an integral part of Ukraine," Yatseniuk said, speaking in English and raising his voice for clarity.
The prime minister, who received a package of financial support worth 11 billion euros ($15 billion) from EU leaders, said his focus remained on achieving a peaceful solution to the conflict, but said that depended on Russia co-operating.
In the past two days, Russia has shown brief signs of being willing to "de-escalate" the situation, with President Vladimir Putin speaking by phone to German Chancellor Angela Merkel and appearing to agree to a "contact group" to mediate the crisis.
However, no sooner have steps been taken towards calming the situation than developments on the ground - such as the vote by the Crimean parliament, or reports of international mediators being detained - have created new uncertainty.
U.S. President Barack Obama took executive steps to impose sanctions including visa bans on individuals in Russia and Ukraine on Thursday, but did not say who might be targeted.
At the same time, EU leaders are considering whether to take similar steps against Russia for its actions in Ukraine. Most diplomats do not expect any hard-hitting measures to be agreed, largely because under EU rules unanimity is required.
But the latest developments on the ground in Ukraine suggested that EU leaders may feel pressure to be seen to do something, and some officials indicated that they could decide to suspend visa discussions with Russia, as well as talks on a new investment and cooperation pact. (Additional reporting by Martin Santa and Justyna Pawlak, writing by Luke Baker, editing by Ralph Boulton)