Barack Obama ramped up pressure on Russian president Vladimir Putin on Thursday by imposing visa restrictions on officials for "threatening the sovereignty of Ukraine” and signing an executive order enabling further sanctions against Moscow.

The White House said the measures were a response to Russia’s "ongoing violation of Ukraine’s sovereignty and territorial integrity”, a reference to its intervention in the southern peninsula of Crimea.

In Brussels, European leaders were holding an emergency meeting to decide whether to impose their own sanctions on Russia. Before the summit, the European Union froze the assets of Ukraine’s ousted Russia-backed leader Viktor Yanukovych and 17 other officials suspected of violations of human rights and misuse of state funds.

Germany made clear that it wanted to give a "political process” a chance before opting to introduce sanctions, and the summit was likely to reserve the option of imposing limited measures against Moscow in the event of no diplomatic breakthrough. Officials were working to establish a "contact group” to mediate in negotiations between Russia and Ukraine.

In Washington, the Obama administration did not say how many people were subject to the visa bans or reference their nationality. However, the targets described as "officials and individuals” are believed to be Russians held responsible for the week-old occupation of Crimea.

The White House said the visa restrictions reflected "policy decision to deny visas to those responsible for or complicit in threatening the sovereignty and territorial integrity of Ukraine” and was in addition to visa denials for figures close to the government of former president Victor Yanukovych, over oppression of protesters.

The president’s press secretary, Jay Carney, said the additional executive order "authorises sanctions on individuals and entities responsible for activities undermining democratic processes or institutions in Ukraine”. He said those included: "threatening the peace, security, stability, sovereignty, or territorial integrity of Ukraine; contributing to the misappropriation of state assets of Ukraine; or purporting to assert governmental authority over any part of Ukraine without authorization from the Ukrainian government in Kiev”.

Carney added: "Depending on how the situation develops, the United States is prepared to consider additional steps and sanctions as necessary.”

In Brussels the emphasis was on diplomacy. The acting Ukrainian prime minister, Arseniy Yatseniuk, pledged that Kiev was prepared to talk to Moscow, saying it was in favour of "cooperation, but not surrender.” Echoing Ronald Reagan’s demand of Mikhail Gorbachev in the 1980s, he said: "Mr Putin, tear down this wall. Tear down this wall of intimidation, of military aggression.”

He accused Russia of being a stumbling block to launching a negotiation over Ukraine. "It seems Russia is reluctant to hold real talks and negotiations,” he said.

Senior EU officials said that the Brussels summit was focusing on how a contact group could be formed. Gemany, France, Britain, Italy and Poland would be represented, if it received a green light. But the officials confirmed that there were splits within the EU over how to proceed. "There are 28 member states. There’s a range of views out there,” said one official.

Dalia Grybauskaite, the Lithuanian president, demanded action to counter Russia’s "open and brutal aggression”. She said: "Russia today is trying to rewrite the borders of Europe after world war two, that is what’s going on. If we allow this to happen, next will be somebody else. We felt in on our skin, it is our neighborhood. This is not only about Ukraine, it is also about the Baltic region. About sanctions, let’s first understand that Russia today is dangerous, that Russia today is unpredictable. It’s not about to be afraid, it’s about to stand up and say we understand it.”

But it was clear that the bigger European powers do not want to take punitive action against Moscow while there is a chance of mediation or negotiation. "Our objective is to deescalate the situation,” said the EU official. "It’s a developing situation. We did get the Russians talking and engaging.”

In Crimea, the regional government took matters into its own hands and announced it would hold a referendum on whether it should officially join Russia on 16 March.

At a press conference in Crimean capital of Simferopol, Rustam Temirgaliev, the region’s deputy prime minister, said the referendum was being held purely to ratify the decision of the Crimean parliament to join the Russian Federation, and the parliament had appealed to Russia to assist with this.

He said the decision, which also gave the go-ahead to the territory to begin preparations to join Russia, "comes into effect from the current moment”. The referendum would be held "only to confirm” the decision.

Temirgaliev said that as of Thursday, the only legal troops on Crimean soil were the Russian army. "Any troops of a third country will be treated as illegal band formations, with all the consequences that entails,” he said.

Meanwhile, 40 unarmed military personnel, on a mission by the Organisation for Security and Co-operation in Europe to try to defuse tensions in the region, were denied entry into Crimean territory.

The OSCE confirmed that its observer mission was "prevented from entering Crimean territory”. It said they were travelling by bus back to the city of Kherson, where they will "hold a meeting and decide on next steps”.

On Wednesday evening, the new leader of the Crimea region, Sergei Aksyonov, said pro-Russian forces had control of all of the peninsula and had blockaded all Ukrainian military bases yet to surrender.

In Brussels, British prime minister David Cameron, French premier François Hollande, German chancellor Angela Merkel and other leaders met on Thursday morning before the main EU summit to discuss a range of possible punitive economic sanctions against Moscow.

Later, the 15-member UN security council was due to hold closed-door talks in New York – the fourth such consultations since Friday.

The US secretary of state, John Kerry, has threatened Russia with isolation "diplomatically, politically and economically” if it does not withdraw from Crimea.

Senior US administration officials said it had taken several days to draw up the executive order authorising visa restrictions and potential economic sanctions on Russian and Ukrainian individuals deemed to have been involved in Russian intervention.

They also insisted their decision predated news of plans for a referendum in the Crimea but was designed to send a "strong message that we intend to impose costs on Russia”.

The US is targeting the "assets and travel” of individuals responsible for events in Crimea but not naming them yet hope it will act as deterrent to future escalation, particularly in other parts of Eastern Ukraine. "There need to be consequences,” said one official in background call with reporters.

The White House was critical of Russian proposals for a secession referendum in Crimea, insisting the government in Kiev has to be involved. "We support the territorial integrity of Ukraine as a whole,” said.

US officials also insist they are not worried about retaliatory sanctions, claiming Russia’s "fragile economy” has "unique vulnerability.”