RIYADH, Saudi Arabia — President Barack Obama spoke with Russian President Vladimir Putin for an hour Friday in an effort to resolve the crisis over Ukraine, the first direct conversation between the two leaders after nearly two weeks of tension.Putin initiated the call, White House officials said. It came after a televised interview in which Obama called for Russia to pull its troops back from the Ukrainian border. In the phone conversation, Obama asked Putin to “put a concrete response in writing” to a proposal the United States has made to resolve the crisis, which involved Russia’s incursion into the Crimean region of Ukraine.The two agreed to a meeting between Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov to discuss the proposal further.The call came as Obama was in Saudi Arabia for a meeting with King Abdullah, where he grappled with another difficult issue: the Saudi desire to have the U.S. provide more arms to rebels fighting the government of Syria. An administration official said Obama was considering allowing the supply of shoulder-held anti-aircraft missiles, along with other options.The missiles would be a potent weapon for the opposition, but the U.S. has resisted providing them, in part out of fear that they could be diverted to terrorist groups that might try to use them against civilian airliners. That is still of concern to the president, another official said.Obama advisers had described the visit to the desert kingdom as a way for him to reaffirm the U.S. commitment to its longtime ally in the Middle East, but the phone conversation with Putin quickly overshadowed the meeting.