Ukraine's former prime minister, Yulia Tymoshenko, urged Western powers Sunday to unite on a hard line against Russian President Vladimir Putin in the Crimea crisis.

"This aggressive striving for power with regard to Ukraine represents not only a danger for the Ukrainian state -- other parts of eastern Europe are also in danger," she told German daily Der Tagesspiegel.

The Russian Embassy in Lebanon beefed up its security measures after obtaining information that an extremist group is monitoring it, the Kuwaiti al-Anbaa newspaper reported on Sunday.

According to the newspaper, an extremist group led by a Jordanian of Chechen origin is plotting to carry out attacks against Russian interests in Lebanon.

NATO said several of its websites were targeted in a "significant" cyber attack on Saturday that was claimed by Ukrainian hackers in what appeared to be the latest bout of virtual warfare linked to the country's crisis.

Spokeswoman for the military alliance Oana Lungescu said on Twitter that the websites had been hit by "a significant DDoS (denial of service) attack", but that it had had "no operational impact".

The Swiss public ministry is looking into suspicions of money-laundering by deposed Ukrainian president Viktor Yanukovych, a spokeswoman said Saturday.

The information from the Money Laundering Reporting Office Switzerland (MROS) had been reported in the Swiss-German press.

U.S. Senator John McCain called for massive military assistance to Ukraine Saturday, while warning that Russia's actions in its former Soviet neighbor could lead to unprecedented measures by the United States and it allies.

"Ukraine is going to need a long-term military assistance program from the United States," McCain told reporters following a U.S. senate delegation visit to the Ukrainian capital, voicing what he said was a personal opinion.

Russia vetoed a Western-backed resolution condemning the Crimea referendum at a U.N. Security Council emergency vote Saturday but China abstained, isolating Moscow further on the Ukraine crisis.

The draft resolution, which says Sunday's referendum would have no validity, got 13 votes in the 15-member council. But it was rejected when permanent member Russia exercised its veto.

Thousands of people rallied in central Moscow Saturday in protest at Russia's intervention in Ukraine, a day before the Crimean peninsula is expected to vote on switching to Kremlin rule.

Waving Ukrainian flags and shouting slogans heard during the Maidan protests in Kiev, the demonstrators urged Russian President Vladimir Putin to pull troops back.

Two people were killed in clashes that broke out between pro-Moscow and pro-Kiev supporters in Ukraine's Russian-speaking city of Kharkiv, police said on Saturday, the second such deadly incident in as many days in the country's tinderbox east.

One pro-Russia protester and a passerby were killed when Ukrainian nationalists opened fire on a group of men trying to storm their headquarters in the city late on Friday, police sources told Agence France Presse.

Ukraine braced Saturday for a highly-charged breakaway vote in Crimea which is likely to see the peninsula annexed by Russia despite the threat of stiff sanctions, and push East-West tensions to breaking point.

An eleventh-hour diplomatic push by the United States to stop the referendum from going ahead failed on Friday, with Moscow refusing to make any decisions until after Crimea votes on a split from Ukraine on Sunday.

Since his return to the Kremlin, Vladimir Putin has grown more powerful than ever before. The strongman, who has ruled Russia for the past 14 years, has muzzled his opponents, check-mated the West on Syria and is now on the verge of annexing Crimea.

On Friday, the Kremlin upped the ante further, hinting it could move forces beyond Ukraine's peninsula to protect his compatriots.