U.S. President Donald Trump (C) looks on as U.S. Secretary of State Rex Tillerson (L) and Saudi Arabia's Crown Prince Muhammad bin Nayef (R) exchange a memorandum of understanding, a commitment by the Gulf states not to finance militant organizations, at the Gulf Cooperation Council leaders summit in Riyadh, Saudi Arabia May 21, 2017. REUTERS/Jonathan Ernst

RIYADH (Reuters) - The United States and Gulf Arab countries will sign an agreement on Sunday to coordinate their efforts against the financing of terrorist groups, a senior White House official said, as U.S. President Donald Trump visits the region.

Saudi Crown Prince Mohammed bin Nayef and U.S. Secretary of State Rex Tillerson will sign the memorandum of understanding in Riyadh, where the six-nation Gulf Cooperation Council (GCC) is holding a summit with Trump.

“It’s the, we hope, farthest reaching commitment to not finance terrorist organizations that (the U.S.) Treasury will be monitoring with each of their counterparts,” said Dina Powell, White House deputy national security adviser for strategy.

“The unique piece of it is that every single one of them are signatories on how they’re responsible and will actually prosecute the financing of terrorism, including individuals.”

The GCC comprises Saudi Arabia, Qatar, Kuwait, Oman, the United Arab Emirates and Bahrain.

Western officials believe that people living in wealthy Gulf countries have been an important source of funding for Sunni Islamist groups fighting in Iraq and Syria, including Islamic State and al Qaeda.