WARSAW, Poland (Reuters) - President Barack Obama met with Greek Prime Minister Alexis Tsipras on the sidelines of the NATO summit in Warsaw on Saturday, the White House said.

The leaders discussed the progress that Greece has made in addressing economic challenges and Obama encouraged Greece to continue implementing economic reforms.

"The president commended Greece's cooperation with Turkey to reduce irregular migration and encouraged longer-term U.S.-Greece defense cooperation," a White House official said.

(Reporting by Ayesha Rascoe; Editing by Catherine Evans)