U.S. President Barack Obama talks with Pope Benedict XVI on Friday at the Vatican. (Haraz N. Ghanbari/Associated Press)

U.S. President Barack Obama met with Pope Benedict XVI at the Vatican on Friday for what White House officials said would be "a very frank discussion."

Several hundred people lined the avenue leading into St. Peter's Square as Obama's motorcade arrived at the Holy See after the president finished participating in the G8 summit in L'Aquila, Italy.

Obama said it was "a great honour" to meet the Pope as the two sat down to a 25-minute meeting in the Papal Library. It was the president's first meeting with the pontiff.

As Obama left the meeting, he told reporters, "We look forward to a very strong relationship between our two countries."

There has been no immediate word about what was discussed during the private session but Obama was expected to brief the Pope on the outcomes of the summit, which included $20 billion US to increase food production, a goal of reducing greenhouse gases by 80 per cent by 2050 and beginning to prepare exit strategies from global economic stimulus plans.

Obama said ahead of the meeting that he would tell the Pope of his concern that the world financial crisis is not "borne disproportionally by the most poor and vulnerable countries."

Pope Benedict issued a major document earlier this week calling for a new world financial order guided by ethics and the search for the common good, denouncing the profit-at-all-cost mentality blamed for bringing about the global financial meltdown.

'Frank discussion'

Before reporters were ushered away from the meeting, Obama told Pope Benedict the three days of meetings had been "very productive."

Ahead of the meeting at the Vatican, White House spokesman Robert Gibbs told reporters the meeting between the two "will be a very frank discussion."

Some of the Obama administration's policies do not match up well with the Roman Catholic Church's views, including policies on issues such as abortion and stem cell research.

Some Catholic activists and American bishops have been outspoken in their criticism of Obama, accusing him of pursuing an anti-life and anti-family agenda.

The Vatican had shared eight years of common ground with former president George W. Bush in opposing abortion, an issue that drew them together despite the Vatican's opposition to the war in Iraq.

Vatican officials had said the Pope wanted to have a constructive dialogue with the president on issues that included peace talks in the Middle East, the environment and relationships with the Muslim world.

'Common ground'

"I think that there's a lot that they agree on that they'll get a chance to discuss," Gibbs said. "We know the Pope has been keenly aware of the president's outreach to the Muslim world. The Pope shares the president's view on reducing the number of nuclear weapons. So I think there's certainly a lot of common ground."

Both Obama and the Pope have been pushing for a two-state solution in the Middle East.

Michelle Obama joined her husband near the end of the meeting and gifts were exchanged with the Pope, officials said.

The president and his family then left the Vatican for a flight to Ghana, where he will deliver a speech on Saturday.

With files from The Associated Press