President Vladimir Putin has approved a draft treaty to make Crimea part of Russia, the Kremlin said on Tuesday, confirming that Moscow plans to make the southern Ukrainian region part of Russia.

It said the president would sign the treaty with Crimea's leader.

Putin signed an order on Monday "to approve the draft treaty between the Russian Federation and the Republic of Crimea on adopting the Republic of Crimea into the Russian Federation".

The order is part of a series of steps to bring Crimea into Russia after voters there approved the move in a weekend referendum that Ukraine and the West have denounced as illegal.

It comes the day after Putin ignored the toughest sanctions against Moscow since the end of the Cold War and recognised the Crimean Peninsula as an "independent and sovereign country".

The brief decree posted on the Kremlin's website on Monday came just hours after the United States and the European Union announced asset freezes and other sanctions against Russian and Ukrainian officials involved in the Crimean crisis.

Tensions between Russia and the West have soared since Russian troops took effective control of Crimea, a strategic Black Sea peninsula last month and supported the Sunday referendum that overwhelmingly called for annexation by Russia.

Putin took a step closer to formally annexing the majority ethnic-Russian region by recognising its independence from Ukraine, opening the way for Russian lawmakers to later endorse its accession.

The US placed sanctions on seven Russian officials, including deputy Prime Minister Dmitry Rogozin, and four Ukrainians accused of usurping Ukraine's territorial integrity, including ousted President Viktor Yanukovich.

"These are by far the most comprehensive sanctions applied to Russia since the end of the Cold War," said a senior US official.

President Barack Obama warned the sanctions would target economic power brokers in Moscow if the Kremlin does not back down from its current course of action.

"If Russia continues to interfere in Ukraine, we stand ready to impose further sanctions," he said.

Asset freezes

Officials noted that the new executive order signed by Obama allows future measures to target "individuals with a great deal of influence over... Russian governmental policy and the economy who do not hold positions in the government."

They would not confirm that was a reference to Russia's vastly rich business tycoons, or oligarchs.

Those targeted under Obama's executive order will see assets and interests in the US or under US jurisdiction blocked and Americans will be barred from doing business with them.

The European Union also unveiled travel bans and asset freezes against 21 Russian and Ukrainian officials, including Vice Admiral Alexander Vitko, the head of Russia's Black Sea fleet.

Obama said a diplomatic solution to the crisis was still possible if Russia pulls its troops back to barracks in Crimea, allows foreign observers to deploy and agrees to negotiate with Ukraine.

British Foreign Secretary William Hague also called for dialogue, stressing that "for the UK and her allies the Crimea remains part of Ukraine".

"Continuing to ignore those calls will bring serious consequences for Russia," he said.

Duma session

Later on Tuesday, Putin is due to address a special joint session of Russia's State Duma, or parliament.

In a statement on Monday, Russia's Foreign Ministry urged Ukraine's parliament to call a constitutional assembly that could draft a new constitution to make the country federal, handing more power to its regions.

It also said the country should adopt a "neutral political and military status," a demand reflecting Moscow's concern that Ukraine might join NATO and establish closer political and economic ties with the EU.

Russia is also pushing for Russian to become one of Ukraine's state languages, in addition to Ukrainian.

In Kiev, Ukraine's new government dismissed Russia's proposal as unacceptable, saying it "looks like an ultimatum."

Russian forces took control of Crimea in late February following the toppling of Yanukovich after deadly clashes between riot police and protesters trying to overturn his decision to spurn a trade and cooperation deal with the EU in favour of cultivating closer ties with Russia.