The European Union is on course to impose travel bans and to freeze the assets of Russian officials and military officers involved in the occupation of Crimea by next Monday if Moscow declines to accept the formation of a "contact group" to establish a dialogue with Ukraine.

A meeting of EU foreign ministers in Brussels on Monday is being seen as an unofficial deadline for the introduction of the sanctions, which would exempt the Russian president, Vladimir Putin, and his foreign minister, Sergei Lavrov, as the EU tries to keep open lines of communication.

The sanctions could be imposed a day after Sunday's referendum in Crimea to allow the Black Sea peninsula to join the Russian Federation.

Ukraine's parliament warned the regional assembly in Crimea on Tuesday that it faces dissolution unless it cancels the referendum, which has been condemned by the EU and the US as illegal. But the Russian foreign ministry said it would respect the result of the vote.

Officials from the EU, US, Japan and Turkey met in London on Tuesday to draw up a list of Russians who could be subject to the sanctions, as Kiev called on London and Washington to live up to their commitments to respect the territorial integrity of Ukraine. Britain, the US and Russia were signatories to the Budapest memorandum in 1994 in which they agreed to uphold the newly independent Ukraine's borders in exchange for the surrender of the nuclear arsenal it inherited from the Soviet Union.

Arseniy Yatsenyuk, the interim Ukrainian prime minister, said: "If you do not uphold these guarantees which you signed up to in the Budapest memorandum, then explain how you will convince Iran and North Korea to give up their nuclear status."

In his second press conference since he fled to Russia, Ukraine's former president, Viktor Yanukovych, decried the actions of the new Kiev government and its western allies but shied away from discussing the de facto Russian occupation of Crimea.

The sanctions will be introduced under what EU leaders describe as phase II of a three-stage plan agreed at their summit last week. The final stage would involve curbs on energy, trade and financial relations if Russian forces move beyond Crimea to the main part of eastern Ukraine.

The number of Russians named in the EU's official journal is expected to be in double rather than triple figures.

David Cameron's spokesman said: "The prime minister is very much linking phase II to the need for dialogue to start in the new few days. We are asking [the officials] to do preparatory work and we still believe there is an opportunity for the dialogue to start and we very much encourage the Russian authorities to start that. The focus [of the sanctions will] be on officials who are closely linked to infringements on Ukrainian sovereignty."

Russians based in London – or oligarchs in Moscow who base their assets in London – will be exempt from sanctions because they are not linked to the "infringements". This approach was criticised by John McCain, the former Republican US presidential candidate, who raised concerns about a secret UK government document which said that "London's financial centre" should not be closed to Russians. The Arizona senator, who described the photographed document as "very unfortunate", told the FT: "Many of the Russian oligarchs would be affected by the sanctions that I advocate. These billionaires with ill-gotten gains from their relationship with Vladimir Putin – it is a bit unsettling that the [UK] government would not want in some way to restrict their lifestyle."

Laurent Fabius, the French foreign minister, told France-Inter radio that the sanctions could be introduced by the end of this week: "There is a set of sanctions that will take effect as of this week and that will consist of freezing the personal assets of Russians and Ukrainians and visa restrictions."

Frank-Walter Steinmeier, the German foreign minister, said a decision would be made by Monday. Berlin, which is heavily dependent on Russian gas, remains cautious.

Meanwhile, the Chatham House thinktank said the referendum on Sunday would give voters no opportunity to break free from Russia. The first question, according to Reuters, will ask voters if they are in favour of the "reunification of Crimea with Russia as a part of the Russian Federation". The second question asks if they are in favour of "restoring the 1992 constitution and the status of Crimea as a part of Ukraine". But the constitution was later abolished.

In an article reported by Reuters, Keir Giles, of Chatham House, wrote: "Even if it [the referendum] were legitimate, the two choices presented to Crimean voters offer them no option for leaving Russian control. The restoration of this [1992] constitution would be a step towards notional independence under Russian control … Those citizens who were content with Crimea remaining part of Ukraine on the same basis as it has been for the last 20 years do not have a voice in this referendum. There is no third option available."