ANKARA - The divergence between Turkey and the United States on the strategy to battle the Islamic State (IS) threat remains largely intact even after US Vice-President Joe Biden's two-day visit in the Turkish coastal city of Istanbul.

Ankara is still unwilling to allow the US-led coalition to gain broader access to key airbase in Adana province in southern Turkey near Syrian border.

"I think, as we continue to have this high-level conversation about how we can act together to combat ISIL in both Iraq and Syria," a senior US official told reporters according to a White House statement.

The unnamed official was also quoted as saying that talks between Turkey and the United States are still ongoing on "the ability of coalition aircraft and other assets to be able to use Turkish facilities".

The official also underlined that Turks are trying to reach a common position with the United States before they open up their platforms "a little bit more".

"Biden returns empty handed to Washington regarding the opening of the strategic air base Incirlik in southern Turkey, Turkish participation in air strikes against ISIL, and wider scale military cooperation," Murat Yetkin, Turkish analyst said.

Turkey declined to be a fully-fledged part of the anti-IS coalition unless Washington agrees to adopt an "integrated strategy" that also targets the removal of Syrian President Bashar al-Assad from power, he said.

However, Washington said the priority is not Assad but rather militant threat from the IS. The United States is also opposed to the idea of establishing a no-fly zone or safe zone inside Syria that has been advocated by Turkey for some time.

Ankara said as long as al-Assad remains in power, militant groups like IS will continue to fill the power vacuum in Syria. It also insists on setting up a safe zone in Syria, protected by a no-fly zone, to take care of refugees inside Syrian territory.

Turkish Deputy Prime Minister Numan Kurtulmus said on Tuesday that Turkey is home to some 1.6 million Syrian refugees and has spent well over four billion US dollars.

He said Turkey might face another big refugee wave if Aleppo falls into the hands of the regime.

Some Turkish analysts believe that may not be the case however.

Lale Kemal, a security analyst, said there is a real concern in Ankara that if the Syrian government can secure Aleppo, many of the refugees in Turkey will go back home.

"That would be a real blow to the Turkish government's plan of continuing to push for Assad to go," she said.

Speaking to party's lawmakers on Tuesday, Turkish Prime Minister Ahmet Davutoglu vowed Turkey would never leave the region to oppressors like Assad.

"Those who hit a city like Aleppo with missiles will be held accountable. Aleppo will be saved one day," he stated.

Turkey, once an ally of Assad, is the fiercest opponent of Assad regime and has been supporting the opposition trying to topple him since 2011.

The disagreement between the United States and Turkey recalls the year 2003 when Turkey was also very reluctant to support the US invasion of Iraq at the time, according to Ali Aslan, a Washington-based Turkish expert.

"During that infamous period, Ankara got into the habit of telling Washington to, 'go home, come back tomorrow'," Aslan said, adding that the Pentagon is just as angry with Ankara as it was then.

The only success story that came out of Biden's meeting with both Davutoglu and Turkish President Recep Tayyip Erdogan was that the two sides were closing on an agreement to train and equip moderate Syrian opposition forces.

The White House statement that quoted an unnamed senior official explained how Turkey and the United States have agreed on doing more over the "train and equip" program of the Syrian opposition.

"The Turks have signed up to host one of those training bases. But we talked about whether there might be some other ways that we can expand cooperation, grow the moderate Syrian opposition faster, make it more effective," the US official said.

Kemal said there is a "convergence of ideas between Turkey and the US over training and equipping moderate Syrian opposition fighters as part of the anti-IS struggle".

She warned however that this US-led program would take longer than expected.

According to Yetkin, both sides have also agreed to boost cooperation against foreign fighters, on intelligence sharing, and border security.

Turkish Foreign Minister Mevlut Cavusoglu revealed late Monday in Parliament that Turkey has expelled 1,100 foreigners as part of battling with foreign fighters.

He also disclosed that some 600 Turkish citizens have joined the IS and more than 100 of them were killed in battles.