Today's meeting in Hannover, Germany, comes on the final day of Obama's two-day visit to push for the conclusion of negotiations on a US-Europe trade deal.
German Chancellor Angela Merkel, British Prime Minister David Cameron, French President Francois Hollande and Italian Prime Minister Matteo Renzi were joining Obama.
In a speech on US-European relations, to be delivered before the meeting, Obama planned to announce the deployment of 250 US military personnel, mostly Army Green Berets, to Syria to assist local forces fighting the Islamic State group.
The move will significantly increase the US presence in Syria and comes a week after Defense Secretary Ash Carter announced the deployment of more than 200 troops to Iraq, where the Islamic State also controls territory, along with the first shipment of Apache helicopters.
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About 50 US special operations forces have been operating in Syria.
The high-level talks between the US and its major European allies come a month after IS militants claimed responsibility for attacks that killed more than 30 people in Brussels, just across the German border.
Topping the agenda today are efforts to counter IS. Other topics include refugee migration, Syria, Russia, Ukraine and Libya.
The leaders are also expected to discuss additional steps by NATO allies to address challenges on Europe's eastern and southern borders, along with efforts to spur agreement on the trade deal.
Obama recently said that failure to plan for the fallout in Libya after the toppling of leader Moammar Gadhafi was his biggest mistake as president.
Libya since has descended into chaos and become a base for IS. The leaders are likely to discuss how they can work with a newly installed, UN-backed unity government to keep IS extremists from further tightening their grip on the North African country.
He discussed assistance to Libya during separate meetings with Cameron and Merkel in recent days. At a news conference yesterday alongside Merkel, Obama said having the unity government "requires us to do everything we can to encourage it.