Turkish President Recep Tayyip Erdogan has denied allegations that authorities in Turkey facilitated an Islamic State (IS) attack on the Syrian border town of Kobane, which has resulted in what a monitoring group has described as a major civilian massacre.

The assault began in the early hours of Thursday morning when a suicide car bomb was detonated at the Mursitpinar border crossing between Turkey and Kobane. Roughly 30 IS gunmen disguised in the uniforms of the Kurdish People's Protection Units (YPG) and Free Syrian Army moved into the town, killing men, women, and children. Turkey's Anadolu Agency released CCTV footage of the bombing, which showed a Toyota pickup headed toward Mursitpinar before detonating.

The Syrian Observatory for Human Rights (SOHR), a UK-based monitoring group, said that it had documented the deaths of at least 120 civilians and that a further 200 had been injured, some critically. Local health services were overwhelmed and the most serious cases were transferred to Turkish hospitals. SOHR described the incident as the second worst civilian massacre committed by IS — which is also known as ISIS, ISIL, and by its Arabic acronym Daesh — and reported that the group had also killed another 26 people in a nearby village.

"Yesterday morning we were told around about 0400 hours that IS went into Kobane, then they went into houses and made a massacre, killing women and children and even babies of only a year old," Mustafa Dogal, a member of Turkey's pro-Kurdish Peoples' Democratic Party (HDP) who headed previous relief efforts for Kobane, told VICE News. "It was a big massacre. We were told that they were wearing YPG clothes to disguise themselves."

SOHR reported that 28 attackers have been killed, but clashes were still ongoing as of Friday afternoon.

Kobane, which is also known by its Arabic name Ayn al-Arab, was almost overrun by a concerted IS offensive beginning in October last year. The extremists were halted with the help of heavy US-led airstrikes, and Kurdish fighters backed by some Syrian rebels eventually retook the town in January after months of fierce fighting.

It is as yet unclear how so many IS militants were able to infiltrate Kobane, even under disguise, and avoid the many YPG checkpoints on its fringes. Some accused Turkey's government — which views the YPG as the Syrian wing of outlawed Turkish insurgent group the Kurdistan Workers' Party (PKK) — of allowing the IS militants to cross the border from Turkish territory.

HDP co-leader Figen Yuksekdag told reporters on Thursday there was a "high probability" that IS had entered from Turkey.

"The Turkish government has supported ISIL for years," she said. "Today's massacre is a part of this support."

Kurdish activists and Syrian state media, which often seeks to portray Turkey as supporting extremists, also claimed that IS had reached Kobane via Mursitpinar.

A picture taken from the Turkish side of the border shows smoke rising after renewed attack by Islamic State in Kobane, Syria, June 25, 2015. (Photo via EPA)

Erdogan angrily branded Yuksekdag's comments "defamation and propaganda."

"No one has the right to link Turkey to terrorist organizations", he said, according to remarks quoted by AFP. "We condemn the heinous attack by the terrorist IS organization that targeted innocent civilians in the city of Kobane."

Turkish officials countered that the IS fighters had actually attacked from Jarablus, an IS-held border town in Aleppo province.

YPG spokesman Rêdûr Xelîl said on Friday that he would "neither confirm or deny" that IS had entered Kobane from Turkey. Dogal noted that he was unsure how the militants had breached the town's defenses.

"Some people are saying that they came through Turkey and some through Jarablus," he said. "It could be anywhere."

Analysts said that there is currently no indication that Yuksekdag's claims were true.

"There's no evidence that the state was involved in this except for the fact that this took place on the border," Royal United Services Institute associate fellow Aaron Stein told VICE News. "But the question everyone has, is how a large convoy of cars, reportedly between two and five, were able to drive through multiple YPG checkpoints on their way into the city. The assumption is that either they could bypass them by traveling via Turkey, pass through with a trusted YPG defector, or that the attackers were sleeper cells left behind when IS retreated."

The HDP gained its first parliamentary seats when it exceeded a 10 percent vote threshold in Turkey's June 7 general election, depriving Erdogan's Justice and Development Party (AKP) of a parliamentary majority and helping to block his plans to alter the constitution and boost his executive powers.

This latest spat highlights the acrimony between the two parties, Stein said.

"This has deepened the Turkish/Kurdish divide that has been growing since even before Kobane, and the feeling amongst a lot of Kurds that AKP and ISIS were synonymous," he remarked.

Turkey is a member of the US-led coalition fighting IS, but it has not taken on a prominent role despite having land borders with both Iraq and Syria, where the extremist group controls large stretches of territory. IS has been able to bring weapons and fighters into its territory through Turkey and smuggle oil out on a fairly large scale. It is unlikely that this could have been achieved without contact between border guards and IS militants and at least some level of collusion from customs and military, but analysts suggest that this is most probably the result of isolated individual breaches.

During the battle for Kobane last year, Kurds in the region often viewed Turkey and IS as collaborators rather than enemies, and accusations the government in Ankara was arming the jihadists were widely seen as fact. Even senior members of the HDP circulated rumors that Ankara had delivered a trainload of tanks to the group.

IS also launched an attack on Syrian government troops in the northeastern city of Hasaka on Thursday, capturing two southern neighborhoods. Dozens died in the fighting and an estimated 60,000 people were displaced, according to the UN Office for the Coordination of Humanitarian affairs (OCHA). Hasakeh is split between Kurds and the Syrian regime, but the YPG were not involved in the fighting.

The two attacks mark a renewed offensive on the part of IS, which has been hit with a series of defeats in Syria since being driven out of Kobane. The YPG and an allied rebel coalition advanced this week to within 30 miles of Raqaa, IS's self-declared capital, reaching the key Brigade 93 base on Monday night after heavy clashes and then pushing IS out of the nearby town of Ain Issa the following day.

Last week, the YPG also took complete control of Tal Abyad, another Syrian-Turkish border town. Tal Abyad is less than 60 miles from Raqqa, and had been used by IS to bring in fighters, ammunition, and supplies from Turkey. The jihadists' closest alternative cross-border route is likely now Jarablus.

The YPG-led advance was one of the first positive signs in the fight against IS for some weeks. The strategy to defeat IS in Syria and Iraq appeared shaky after the group overran historic Palmyra in Syria and captured Ramadi in neighboring Iraq.