US Vice President Joe Biden warned Russia on Tuesday that the US and Europe could impose further sanctions, denouncing the absorption of Crimea into the Russian Federation as "nothing more than a land grab."

"It's a simple fact that Russia's political and economic isolation will only increase if it continues down this dark path," said Biden was visiting Poland to reaffirm US support for NATO allies bordering Russia.

Both the EU and US have agreed asset freezes and travel bans against politicians and officials from Russia and Ukraine who were deemed to have played a part in the lead up to Sunday's referendum in which Crimea voted to secede Ukraine.

The US warned of an escalation of the measures on Tuesday after Putin signed a treaty approving Crimea's absorption into the Russian Federation, but the EU has yet to reach a consensus on further sanctions.

Chancellor Angela Merkel spoke to US President Barack Obama by telephone on Tuesday, with both agreeing on the need for close cooperation between the EU and US.

'An unacceptable blow'

The leaders agreed that: "The unilateral declaration of independence by Crimea and the beginning of its incorporation into the Russian Federation today are an unacceptable blow to the territorial integrity of Ukraine," said Merkel's spokesman Steffen Seibert.

"The chancellor and president regard the targeted measures agreed by the EU and US against individuals as correct in this situation."

The US imposed visa bans and asset freezes on 11 Russians on Monday, while the EU announced similar sanctions against 21 individuals.

Russia's response to the first phase of sanctions has been dismissive. Deputy Prime Minister Dmitry Rogozin - who is one of those on the sanctions list - said on Twitter that the steps were weak and meaningless.

'Put us on your list!'

Russia's parliament, the Duma, on Tuesday went as far as to urge the West to impose the same penalties on hundreds of lawmakers who voted for the inclusion of Crimea in the Russian Federation.

"We propose that Mr. Obama and the ... Eurobureaucrats include all State Duma deputies who voted in favor of this resolution on the list of Russian citizens affected by US and EU sanctions," said the declaration, which was adopted by a 353-0 vote.

EU leaders are to meet in Brussels on Thursday and are expected to discuss a second stage of sanctions; that of adding names to the list. A third level would target wider finance and trade.

While several countries, particularly former Soviet states, are keen for more sanctions, others are in a quandary over whether the measures will be effective, and the potential damage to trade relations.

Shortly after Biden addressed reporters in Warsaw, US President Obama invited leaders of the G-7 and European Union to meet about the crisis in Ukraine, according to the White House.

rc/ccp (AFP, AP, dpa, Reuters)