The United States is seeing a dramatic surge in oil and gas production and could overtake the world's biggest producers, Russia and Saudi Arabia, in another decade, a U.S. official said here Tuesday.

"Some of the numbers are eye-popping," Daniel Sullivan, commissioner in Alaska's department of natural resources, told a panel of experts at the International Economic Forum of the Americas in Montreal.

Russia's arms export agency said Wednesday its deliveries to Syria were in line with U.N. regulations but declined to comment specifically on U.S. claims that it was sending attack helicopters to the regime.

Rosoboronexport "does not supply weapons and military technology in contradiction with U.N. Security Council security requirements and other international agreements," an agency spokesman told the state-run RIA Novosti news agency.

Iran on Tuesday welcomed a Russian proposal on holding an international conference on Syria aimed at saving a faltering peace plan for that country's conflict.

"We support any method which helps positive talks between the government and the opposition in an appropriate environment, and also suitable reform actions for realizing the people's ambitions," Iranian foreign ministry spokesman Ramin Mehmanparast told reporters at a weekly press briefing.

A Saudi trade body cancelled a Tuesday meeting with a Russian delegation of businessmen in protest at Moscow's position on the Syrian uprising, a top official said.

"The Chamber of Commerce and Industry in Riyadh has cancelled a scheduled meeting with a delegation of Russian businessmen in solidarity with the Syrian people," said Abdel Rahman al-Juraissi, president of the Saudi trade body.

President Vladimir Putin vowed on Tuesday not to let Russia be weakened by social shocks as tens of thousands of demonstrators gathered in Moscow to protest his historic third term.

"We cannot accept anything that weakens our country or divides society," he said in a televised address on the Russia Day national holiday.

Tens of thousands of protesters chanting "Russia Will be Free" rallied in Moscow on Tuesday against President Vladimir Putin's third term despite a police crackdown on their leaders a day earlier.

Flag-waving crowds made their way down leafy boulevards from Moscow's central Pushkin Square as the march began with police putting the numbers at around 18,000.

The United States on Monday voiced concern after Russian police raided the homes of top protest leaders ahead of a planned mass rally in Moscow against President Vladimir Putin.

"The United States is deeply concerned by the apparent harassment of Russian political opposition figures on the eve of the planned demonstrations on June 12," State Department spokeswoman Victoria Nuland told reporters.

EU foreign policy chief Catherine Ashton met senior officials from world powers on Monday to prepare for talks in Moscow this month on Tehran's contested nuclear drive.

Ashton and officials from the so-called P5+1 group -- Britain, China, France, Russia and the United States plus Germany -- met in a Strasbourg hotel "to prepare the ground for the negotiations in Moscow," an EU spokesman said.

France said Monday it will hold talks with Russia on its idea of an international Syria conference including Iran and urged the new head of the Syrian National Council to unite the opposition.

"We will have new contacts with Russia this week on the subject," a ministry spokesman, Vincent Floreani, told a press conference.

Russian Foreign Minister Sergei Lavrov will visit Iran on Wednesday to discuss the upcoming Moscow talks on Tehran's nuclear program as well as the Syria crisis, his ministry said.

"The focus of attention... will be on the situation in the Middle East and North Africa, with the emphasis on the current transformation processes taking place in the Arab world and Syria," news agencies quoted a foreign ministry official as saying Monday.