A Ukrainian serviceman has been killed in an attack at a military facility in the Crimean capital Simferopol, the defence ministry has said.



Al Jazeera's Paul Brennan, reporting from Kiev, said news of the soldier's death had been received with "shock" and that Tuesday's attack had escalated the situation, which he said was "hugely tense".



Our correspondent said Ukrainian authorities described the dead soldier as a warrant officer, but a Crimean news agency, which cited a source in Ukraine's interior ministry, called the victim a self-defence fighter.



It said self-defence fighters were shot by a sniper from an uncompleted building opposite a Ukrainian military base, which is flying a Russian flag. The defence ministry described those behind the attack in the peninsula as "armed masked men".

Al Jazeera's Nick Schifrin, reporting from Simferepol, said there was evidence of bullet marks on the uncompleted building opposite the army base, suggesting some shots had been fired.



"We saw what appeared to be Russian snipers leaving positions from a separate building that overlooks the base," he said.

A statement from the Ukrainian defence ministry said a decision had now been taken by the commander-in-chief and the acting defence minister to allow Ukrainian troops to use force in self defence.



Referendum

The attack comes as, defying Ukrainian protests and Western sanctions, Russian President Vladimir Putin signed a treaty in Moscow on Tuesday making Crimea part of Russia again.

Ukraine's Western-backed prime minister said after the incident that his country's conflict with Russia was entering a "military stage", though Putin said there were no plans to seize any other part of Ukraine.

In a fiercely patriotic address to a joint session of parliament in the Kremlin, punctuated by standing ovations, cheers and tears, Putin said Crimea's disputed referendum vote on Sunday, held under Russian military occupation, had shown the overwhelming will of the people to be reunited with Russia.

As the Russian national anthem played, Putin and Crimean leaders signed a treaty to make Ukraine's region part of the Russian Federation, declaring:

"In the hearts and minds of people, Crimea has always been and remains an inseparable part of Russia."

Parliament is expected to begin ratifying the treaty within days.

'Economic isolation'

Putin has said that Sunday's vote was in line with international law, reflecting Ukraine's right for self-determination.

He pointed at the example of Kosovo's independence bid, supported by the West, and said that Crimea's secession from Ukraine repeats Ukraine's own secession from the Soviet Union in 1991.

He denied Western accusations that Russia invaded Crimea prior to the referendum, saying Russian troops were sent there in line with a treaty with Ukraine that allows Russia to have up to 25,000 troops at its Black Sea fleet base in Crimea.

US Vice President Joe Biden and Polish Prime Minister Donald Tusk rejected Putin's remarks during a press conference held in Warsaw.

At the beginning of a two-day trip to Europe that also includes a visit to Lithuania, Biden assured Poland that the US is a "steadfast ally" and that it will protect NATO allies on Russian borders, if needed.

"Russia's economic isolation will only increase if it continues down this path, and we will see additional sanctions by the US and the EU," Biden said.