Turkish officials have denied reaching a new agreement to let the US use its Incirlik airbase in the fight against Islamic State (Isis) militants, in comments which ran contrary to a statement made by US national security adviser Susan Rice on Sunday.

The Turkish prime minister’s office said an agreement had been reached to train moderate Syrian rebels on Turkish soil, but it was not yet clear “where [and] in what way” that would happen.

On Sunday, the US said Turkey had agreed to let US and coalition forces use its military bases, including the key Incirlik airbase close to the southern city of Adana, 100 miles from the Syrian border.

But Turkish officials insisted that no decision had been taken on Incirlik.

Serhat Güvenç, lecturer in international relations at Kadir Has University in Istanbul, said that increased Turkish involvement in the fight against Isis had not come as a surprise, but had come earlier than expected. “While Turkey did not have much to bargain with to resist a more active participation much longer, recent government statements underlining Turkey’s main goal – the ousting of [Syrian president] Bashar al-Assad – made it seem like Ankara would resist a while longer,” he said. “This takes Turkey’s place in the international coalition to a higher level.”

Washington has long been pressing the Turkish government to take on a more decisive role in the fight against Isis militants, who have continued their incursions into both Syria and Iraq and now control large swaths of territory in both countries. A violent Isis onslaught against the Syrian Kurdish town of Kobani at the border with Turkey has displaced more than 100,000 people so far.

On Sunday unnamed US officials said Ankara had agreed to open its airbases and allow the training of moderate Syrian rebels on Turkish soil. According to Turkish government sources cited by Associated Press, about 4,000 opposition fighters will be trained in Turkey, after being screened by Turkish intelligence. A Turkish daily newspaper reported that Kurdish PYD fighters, closely linked to the outlawed Kurdistan Workers’ party (PKK), would be among the fighters.

Idris Nassan, deputy foreign minister in the Kurdish administration for Kobani, did not confirm this claim, but demanded unity in the fight against Isis: “We should all unite against the terrorist threat of Isis, and Kurdish fighters, whoever they are, should be an integral part of that effort.”

Meanwhile, fighting in Kobani raged on through the night, with Isis keeping the town centre under heavy artillery and mortar fire. However, neither side was able to gain any more ground, according to Nassan. “On Sunday around 5pm Isis restarted heavy attacks on Kobani, but the fighters of the YPG [Kurdish militia defending Kobani] were able to hold their positions against them,” he said, speaking over the phone. On Monday morning a vehicle loaded with explosives injured two YPG fighters, he added. “Isis receives constant reinforcements in fighters and arms. Though the situation in Kobani is slightly better, the city is under constant threat.”

He added that Turkey’s move might yet spell hope for the beleaguered district. “It’s not too late for Kobani. This might change the situation for us.”

Over the past weeks Turkey has come under heavy criticism for not opening its borders to military aid for Kurdish forces in Kobani. Though Ankara allowed refugees from the Kurdish Syrian district to enter Turkey, no fighters were allowed to leave for Syria through Turkish border posts, and government officials have repeatedly stated that Isis and the PKK were “one and the same”. Many Kurds in Turkey perceive this unwillingness to help the PYD in Kobani as an indirect admission that in a choice between Isis and the PKK, Ankara would choose the former. At least 38 people were killed during violent clashes between Kurdish factions and security forces in several cities in the predominantly Kurdish south-east last week.

Serhat Güvenç said that by increasing its participation in the coalition’s efforts to “degrade and destroy” Islamic State, the Turkish government gained a stronger position to impose its own ideas in Syria: “Without broader participation Turkey could not hope to have the final say on what is to happen in Syria.”

Turkey’s president, Recep Tayyip Erdoğan, has repeatedly said the priority for Turkey is the ousting of Assad. Speaking at an Istanbul university on Monday, Erdoğan said: “We will fight against Isis with the same conviction as before, but we have certain conditions. One: there needs to be a no-fly zone. Two: there needs to be a buffer zone [inside Syria]. Three: we have to train [opposition fighters]. Four: the Syrian regime has to be targeted. You cannot solve this situation in Kobani alone. There are many Kobanis in Syria. If there is Kobani today, there is Aleppo, Hasakah and Mosul tomorrow.”

Observers pointed out that Turkey’s more active participation in the fight against Isis might mean a heightened security threat within its own borders. According to Turkish media reports, Turkish police confiscated “large amounts” of explosives, including several suicide vests, in the south-eastern province of Gaziantep this weekend. Police are reportedly investigating whether the seized material was part of a possible Isis operation in Turkey.