ISTANBUL: U.S. Vice President Joe Biden wrapped up a key visit to Turkey Sunday without a breakthrough on military cooperation in the Syrian crisis but with officials confident the talks brought their positions closer.

Biden’s departure came as rebel groups mounted an offensive against two Shiite villages in Aleppo province, while the jihadi group ISIS downed a regime warplane in Deir al-Zor province to the east, according to the Britain-based Syrian Observatory for Human Rights.

During his three-day trip to Istanbul, Biden held several hours of talks with both Turkish President Recep Tayyip Erdogan and Prime Minister Ahmet Davutoglu on the crises created by the capture by ISIS of parts of Iraq and Syria.

After four hours of talks with Erdogan Saturday, the pair did not announce any new Turkish contribution to the coalition mobilized to fight ISIS but Biden insisted the bilateral relationship was “as strong as ever it has been.”

U.S. officials described the talks as the latest step forward in finding common ground on Syria after Erdogan’s meetings with U.S. President Barack Obama at the NATO summit in September and with Biden himself on the sidelines of the U.N. General Assembly.

A senior U.S. official insisted that the lack of a major announcement should not be seen as a failure and the two sides had a far better mutual understanding of their respective positions than a few months ago.

“We are in complete agreement that ISIL needs to be defeated,” said the official, using another name for the group. “We agree on the major objectives of the strategy.”

But the official acknowledged the two sides still needed to work on their military cooperation in the crisis.

This could include use of Turkey’s Incirlik air base if Turkey’s conditions were satisfied, the official said. Turkey has repeatedly conditioned its support for the coalition on establishing a security zone, backed by a no-fly zone, inside Syria on the Turkish border and on agreement on a coherent international strategy to topple President Bashar Assad.

The official said progress was being made between Turkey and the United States, describing exchanges as “more evolved and dynamic.”

Biden, at his final news conference with Erdogan Saturday, tellingly emphasized they had discussed a “transition” in Syria that does not include Assad.

The warm words by Biden and Erdogan about the state of relations appeared to end a spat that erupted when the U.S. vice president suggested in a lecture that Turkey’s policy on Syria had helped trigger the rise of ISIS.

Biden’s office announced that Washington would provide an additional $135 million for humanitarian aid for Syria, some of which would go to refugees living in Turkey.

In Syria, a coalition of jihadis from the Nusra Front, Islamist militias and mainstream rebel groups launched an attack aimed at seizing two Shiite villages in Aleppo province. The villages of Nubl and Zahraa have been under rebel siege for a year and a half.

Late Saturday, the rebels and their allies from the Al-Qaeda affiliate launched a major assault aimed at taking the villages and seized territory on their outskirts.

“Fierce clashes have raged since midnight on the edges of Nubl and Zahraa,” the Observatory said.

Observatory director Rami Abdel-Rahman told AFP it was the first time that the Nusra Front had made advances in the area.

The Observatory added that at least eight rebels and several pro-government fighters have been killed in the clashes.

The main forces defending Nubl and Zahraa are pro-regime militiamen and fighters from Lebanon’s Hezbollah.

Elsewhere, ISIS fighters shot down a government warplane over Deir al-Zor Sunday, the Observatory said, marking the first such known downing of regime aircraft by the jihadis.

ISIS supporters said in September that they downed a regime jet in the city of Raqqa, although the incident remained murky.

The aircraft in Deir al-Zor was downed after the air force carried out some 20 strikes against jihadi positions in the province.

In the town of Albukamal on the Iraqi border, six ISIS militants were killed in a roadside bomb attack, the Observatory said. The jihadists have come under sporadic, hit-and-run attacks since their takeover of the area this summer, usually carried out by small cells of tribesmen and rebel fighters.

Separately, three militants from the group were killed in a coalition airstrike on their position near Albukamal early Sunday, the Observatory said.

Also, the Observatory reported 25 regime loyalists killed in fighting and rebel ambushes in Zibdine east of Damascus Saturday.

Rebel-held Zibdine is located in the eastern Ghouta area, an insurgent stronghold under army siege for more than a year.