Defence, counterterrorism cooperation and the IS jihadist group were the focus of US President Donald Trump's talks in Rome on Wednesday with Italian head of state Sergio Mattarella and Premier Paolo Gentiloni, the White House said in a statement.

The closed-door talks "discussed the alliance between the United States and Italy, as well as priorities in the areas of defence cooperation, counterterrorism, and efforts to deny terrorists safe havens from Mali to Libya to Iraq to Afghanistan," the statement said.

During the talks, Trump thanked Italy for efforts in fighting terrorism, especially its participation in the US-led global coalition against IS, in which 68 countries are taking part.

He also thanked Italy for "its active role in Iraq and Afghanistan" and praised its "important diplomatic efforts" to end the violence in conflict-wracked Libya, the statement added.

The White House said Trump also "reaffirmed transatlantic unity" on the need to "hold Russia accountable" for its 2014 annexation of Crimea and the continuing conflict in eastern Ukraine, according to the statement.

Trump also stressed the importance of convincing Russia to fulfil its commitments under the Minsk Agreements (a ceasefire deal between the Ukrainian army and Russian-backed separatist rebels signed in Minsk in February 2015), said the statement.

Also on the agenda in Trump's meetings with Mattarella and Gentiloni was the G7 Summit taking place in Taormina, Sicily on Friday and Saturday, the statement concluded.

International issues were also on the agenda at a private meeting between Trump and Pope Francis earlier on Wednesday, the Vatican said.

Trump's talks in Rome with top officials came during the third leg of Trump's first official trip abroad since taking office.

The four-nation tour, aimed at uniting Muslim, Jewish and Catholic leaders in the fight against Islamist terrorism, has included stops in Saudi Arabia, Israel and the Palestinian territories.

Trump's nine-day trip also includes a one-day visit to Brussels on Thursday, where he will attend a NATO summit.

The deadly bombing of Britain's Manchester Arena this week will bring new urgency to Trump's push for NATO allies to commit to a greater role in counterterrorism and the fight against IS in Syria and Iraq, according to observers.