Ankara says Assad regime trying to drag it into a 'nightmare scenario' as cross-border violence threatens to escalate

Syria and Turkey exchanged angry recriminations on Sunday after twin car bombings on the Turkish side of the border killed 46 people, in an attack that has further raised tensions between the neighbours and fuelled fears that the conflict is spreading across the region.

Recep Tayyip Erdogan, the Turkish prime minister, accused Syria of trying to "drag us into a nightmare scenario". Damascus denied that it was involved in Saturday's incident in Reyanhli, in the Hatay region, a hub for Syrian refugees and armed rebels. Nine Turkish citizens were reportedly arrested and confessed.

As a pro-opposition monitoring group estimated that 80,000 people had now been killed in the 26-month conflict, the exchanges provided alarming evidence of the destabilising effect of the crisis.

"Those who attack Turkey will be held to account sooner or later," Erdogan pledged.

In response Omran al-Zoubi, Syria's information minister, told reporters: "Syria didn't and will never undertake such acts because our values don't allow us to do this. It is Erdogan who should be asked about this act. He and his party bear direct responsibility.

"The Turkish government … turned the border areas into centres of international terrorism, as it is still facilitating the arrival of arms and explosives, improvised explosive devices, cars, money and murderers to Syria."

But Turkey's accusation left little room for doubt. "This incident was carried out by an organisation which is in close contact to pro-regime groups in Syria and I say this very clearly, with the Syrian mukhabarat [intelligence service]," the interior minister, Muammer Guler, told Turkish TV.

The main anti-Assad opposition group, the Syrian National Coalition, condemned the "heinous terrorist attacks" in Reyhanli, calling them "an attempt to take revenge on the Turkish people and punish them for their honourable support for the Syrian people".

Syrian mortar rounds have fallen over the border before – five civilians were killed when shells hit the Turkish town of Akacakale last October – but if the allegation is true then the latest explosions would be Turkey's largest death toll related to the Syrian crisis. Nato has already deployed Patriot missile batteries to protect border areas from the threat of Syrian missile attacks.

Turkish media suggested there would now be pressure on Erdogan, and in turn on Barack Obama, to intervene. The Turkish leader will be in Washington for talks this week, as will David Cameron – who discussed Syria with Vladimir Putin in Moscow last Friday.

Turkey, Lebanon, Iraq, Jordan and Israel have all been affected by or become involved one way or another in Syria. Further afield Iran, a close ally of Assad, is also an active participant. The US and Britain both condemned the bombings. Sir Nigel Sheinwald, a former British ambassador to the US, said it was impossible to imagine that US or British troops would be deployed to Syria. But he told the Murnaghan programme on Sky News that the rebels might be given more help, perhaps through the establishment of a border safe zone, while Russia was persuaded to back peace talks between Assad and the rebels.

The figure of 82,257 dead came from the Syrian Observatory for Human Rights. The UK-based watchdog said these included 34,473 civilians – among them 4,788 children and 3,049 women.

The SOHR also recorded the deaths of 16,687 rebel fighters, including defected military personnel. It said 16,729 soldiers and more than 12,000 shabiha (pro-regime militia) and regime informants have also died. Another 2,368 bodies had been found across Syria. The observatory said its toll does not include more than 10,000 people missing in detention in regime jails, or some 2,500 pro-regime prisoners kept in rebel hands.

The UN says that some 1.4 million people have fled the country while 4.2 million other Syrians are internally displaced. Elsewhere in Syria, weekend reports described heavy shelling of the besieged rebel-held Qusair area south of Homs, where fighters of Lebanon's Hezbollah are backing the Syrian army. Qusair is strategically important because it links Damascus with the coastal region, where regime loyalists are concentrated.

Sana, the state news agency, reported that the main highway from the capital to Deraa, the largest town in southern Syria, was now safe after the army eliminated large numbers of "terrorists" in the Khirbet Ghazaleh area. It had been billed by the rebels as a key position.

Rebel sources said weapons that should have been delivered from Jordan had not materialised. Rebels also said on Sunday they had released four Filipino UN peacekeepers they were holding "for their own safety" after clashes last week with government forces put them in danger. A spokesman for the Yarmouk Martyrs Brigade said the four were handed over at a border checkpoint where the Jordanian and Israeli borders join the Golan Heights.