US President Barack Obama is set to visit Hanover, Germany on Sunday to hold talks with German Chancellor Angela Merkel, one of his closest allies in dealing with a shaky global economy and security crises in the Middle East and Ukraine.

It will be the last stop on a six-day foreign journey where Obama has sought to shore up US alliances he views as key to grow trade, defeat Islamic State militants, and offset Russian aggression in Ukraine and Syria.

Obama, who is in the last nine months of his presidential term, spent three days in London where he urged Britons to remain part of the European Union in a June referendum, a vote that could send shockwaves through the economy.

Earlier in the week, he met with Gulf leaders in Riyadh to try to allay fears that Washington had become less committed to their security.

In Hanover, he will tour and speak at a massive industrial trade fair with Merkel. The leaders want to breathe life into a US-European free trade agreement which supporters say could boost each economy by some $100 billion.

Their push comes at a time when many Europeans and Americans alike are deeply suspicious the deal could cost jobs and affect standards.

"But time is not on anyone's side at the moment," said Heather Conley, a former State Department official in the George W. Bush administration, now with the Center for Strategic and International Studies think tank in Washington.

Leaders are trying to wrap up complex talks on the Transatlantic Trade and Investment Partnership (TTIP) before Obama, a Democrat, leaves office on Jan. 20.

Getting a sign-off from the Republican-controlled US Congress in the heat of an election campaign will be a tall order. Obama has yet to secure approval for the sweeping Trans-Pacific Partnership trade pact, which is at a much more advanced stage.

"Getting trade deals done is tough, because each country has its own parochial interests and factions. And in order to get a trade deal done, each country has to give something up," Obama said at a London event on Saturday.

In Hanover, thousands of protesters holding placards with slogans like "Stop TTIP" marched on Saturday to express their opposition to the deal.

Before Obama returns to Washington late Monday, he and Merkel will get together with Cameron, French President Francois Hollande, and Italian Prime Minister Matteo Renzi to talk about beefing up intelligence sharing after recent attacks in France and Belgium.

The leaders are set to talk about how best to find a political settlement in Syria. Hundreds of thousands of people have fled the war-torn region for Europe, where countries have grappled with the flood of refugees. – Reuters