The top European Union executive warned on Wednesday of economic "consequences" for Ukraine if the country does not review disputed elections there, a stance that could strain relations with Russia ahead of a key summit with President Vladimir Putin.

Putin has strongly backed Ukrainian Prime Minister Viktor Yanukovych (search), who official vote counts have shown winning the weekend's run-off election for president.

The opposition, however, said the vote was rigged and that its candidate, Viktor Yushchenko (search), was the winner, and tens of thousands of protesters have demonstrated demanding the official results be overturned.

Western election observers have reported voting abuse in favor of Yanukovych, and European Commission President Jose Manuel Barroso (search) warned Wednesday that "there will be consequences, if there is not a serious, objective review" of the results.

At risk might be around $1.31 billion the bloc has given or committed to Ukraine since 1991, in development and economic aid and possible visa-bans on politicians and officials.

Barroso said the EU would make its position clear" with Putin at Thursday's EU-Russia summit in The Hague, the Netherlands.

"It is our duty to say we are not satisfied with the way the elections took place in Ukraine," he said.

Yushchenko wants ties with NATO and the European Union, while Yanukovych advocates closer Russian ties.

Putin has sought to shore up Russia's influence in Ukraine and former Soviet republics, and he congratulated Yanukovych over the election results.

Pressure mounted throughout the day on the Ukraine government.

At the Vatican, Pope John Paul II (search) told Ukrainians he was praying in a "special way" for Ukraine. As soon as the pontiff started speaking in Ukrainian, about 100 pilgrims from that country stood up and started singing a religious song with patriotic overtones.

EU foreign policy representative Javier Solana (search) said he would consider leading a high-level delegation to Ukraine to see if he could help resolve the tense political standoff but said he had "doubts" now was the time to go.

Polish Liberal Democrat Grazyna Staniszewska said if Solana did not go, the parliament should send its own team and consider pressing EU governments to impose sanctions.

"We must send a new observer mission to prevent a bloodbath," she said.

Ukraine's ambassador to the European Union was called to appear before the European Parliament on Wednesday to defend the conduct of the disputed presidential election.

NATO and many EU nations also summoned Ukraine envoys to hear complaints about the outcome of the second round.

NATO Secretary-General Jaap de Hoop Scheffer (search) called on the Ukraine government "to avoid any use of violence and to allow" legal claims relating to the elections run their course "fully and fairly," NATO spokesman James Appathurai said.