Russian President Vladimir Putin called President Barack Obama today to talk about a U.S. proposal to resolve the conflict over Ukraine and the two leaders dispatched their top diplomats to continue discussions.

Obama asked for a written response from the Russian leader to the proposal that Secretary of State John Kerry presented to Russian Foreign Minister Sergei Lavrov in the Hague earlier this week, according to a statement from the White House.

Obama told Putin that a diplomatic solution “remains possible only if Russia pulls back its troops and does not take any steps to further violate Ukraine’s territorial integrity and sovereignty,” the White House said. Obama took the call from Riyadh, Saudi Arabia, the final stop on a week-long trip that has been dominated by the Ukraine crisis.

In previous meetings with Lavrov, Kerry called for talks between Russia and the Ukrainian government with international participation, and sending monitors into Ukraine, including Crimea. Russia would be able to maintain its bases on the Black Sea peninsula as long as Ukraine’s sovereignty was respected.

Over the course of three days in Europe, Obama sharpened the U.S. response to Russia’s incursions into Ukraine, elevating the crisis to a test of the internationalist framework of foreign affairs he favors. At a summit originally intended to discuss nuclear security and through an emergency meeting of the Group of Seven nations, the U.S. and its allies presented unified opposition to Russia’s actions and the threat of escalating sanctions.

U.S. officials have warned that a buildup of Russian troops on Ukraine’s eastern border suggests that Russia may seek to carve off more of Ukraine’s east and south. Obama urged Putin to pull Russia’s forces back from Ukraine’s frontier.

Obama’s statements as he moved through Europe clarified a U.S. reply to Putin that the administration already was signaling. The annexation of Crimea is to be met with existing sanctions and diplomatic isolation. Further land grabs in Ukraine would trigger sanctions that target more vital sectors of the Russian economy. And a military response would be reserved for Russian action against a North Atlantic Treaty Organization ally, such as Poland or one of the Baltic republics.