Vice President Joe Biden met with Turkey's President Tayyip Erdogan for discussions expected to include Turkey's role in the coalition against the Islamic State. (Reuters)

After an extended period of public estrangement and sniping, the United States and Turkey have made up and say they are heading toward close cooperation on defeating the Islamic State in Syria and Iraq, and eventually seeing the end of the Syrian government of President Bashar al-Assad.

Vice President Biden, who met for four hours here Saturday with Turkish President Recep Tayyip Erdogan, called the bilateral relationship “as strong as it has ever been.” Erdogan described Biden’s three-day visit as “very meaningful.”

The visit did not result in any firm new agreements over such sensitive issues as Turkey’s long-standing call that protected buffer zones be created inside Syria along the Turkish border, or U.S. requests to use Turkey’s Incirlik Air Base to fly bombing missions against the Islamic State in Syria and Iraq. But officials from both governments said their cooperation was rapidly growing, and neither has ruled out meeting the needs of the other.

Senior U.S. and Turkish officials said their rapprochement has intensified since summer with a series of high-level meetings, including a near-constant flow of visits to Turkey by senior U.S. diplomatic and military officials in the weeks before Biden’s trip.

“The conversation has evolved and is quite dynamic,” said a U.S. official traveling with Biden who spoke on the condition of anonymity about the private talks. The vice president and Erdogan “didn’t sit there and sign their names on the bottom lines. . . . But they came to much greater clarity about where we need to go from here.”

Vice President Joe Biden, left, speaks during a joint press conference with Turkish President Recep Tayyip Erdogan at Beylerbeyi Palace Saturday in Istanbul. (Bulent Kilic/AFP/Getty Images)

The official added: “Some additional questions went unresolved, and both our systems have to noodle over those. . . . But we have a much better understanding of the needs and constraints” each government is under.

Turkey, which is hosting about 1.6 million Syrian refugees, has expressed concern that President Obama’s decision to launch airstrikes against Islamic State militants controlling much of northern and eastern Syria, will push the insurgents into Turkish territory. At the same time, Erdogan and Turkish Prime Minister Ahmet Davutoglu have charged that, with the U.S. bombing campaign against the militants, Obama has lost interest in Ankara’s primary goal of helping moderate Syrian opposition rebels to oust Assad.

While the Syrian war has raged for more than three years, Turkey has called for U.S. airstrikes against Syrian government forces. The government’s anxiety has grown as the rebels have been driven out of most of their remaining strongholds in northwestern Syria by Assad’s military and other militant groups, while the rest of the world has been transfixed by the advance of the Islamic State.

The United States opposes a buffer zone, and especially the use of U.S. air power to defend one. But the Obama administration has expressed some interest in using rebels forces being trained in a covert CIA program to establish safe havens along the border with help from Washington and Ankara. The administration recently decided to expand that program, even as Congress has authorized the U.S. military to begin its own training program for rebels to take over territory farther east that is being held by the Islamic State.

“We talked about whether there might be some other ways that we can expand cooperation and grow the Syrian opposition,” the administration officials said. Discussions are “moving in a very positive direction in getting a fuller understanding about how we might accelerate” improving rebel capabilities.

Earlier this year, the administration had raised the volume on long-standing complaints that Turkey was allowing foreign fighters, particularly from Europe, to cross its border to enter the ranks of the Islamic State and also that it was turning a blind eye to militant smuggling from oil fields it had seized inside Syria. In recent weeks, however, U.S. officials have praised the Turks for significant improvement in both areas.

When militant forces advanced on the border town of Kobane — with the fighting in full view of television cameras positioned on nearby hills inside Turkey — Ankara felt it was being “demonized,” in the words of one senior Turkish official, because it was not sending troops to help the town’s mostly Kurdish defenders. A U.S.-Turkish agreement last month to facilitate the transport and entry of Iraqi Kurdish fighters into Kobane contributed to the new rapprochement between the two governments that Biden’s trip was designed in part to solidify.

As part of his visit, Biden announced that the United States would provide nearly $135 million in additional humanitarian assistance “to help feed civilians” from Syria who have taken refuge in Turkey. “Turkey is carrying a heavy humanitarian burden,” Biden said as the two leaders spoke to the media following their discussion.

“We want to continue our cooperation with the United States by strengthening it,” Erdogan said.