Nato is ready to defend Turkey, the alliance's top official said on Tuesday, in a direct warning to Syria after a week of cross-border artillery and mortar exchanges dramatically escalated tensions between the two countries.

Ankara has sent additional fighter jets to reinforce an air base close to the frontier with Syria where shells killed five Turkish civilians last week, sparking fears of a wider regional crisis. Syria has defended its shelling of neighbouring Turkey as an accidental outcome of its civil war.

The comments by Nato secretary general Anders Fogh Rasmussen were the strongest show of support to Turkey since the firing began on Wednesday – though the solidarity is largely symbolic.

Nato member Turkey has sought backing in case it is attacked, but despite publicly supporting Syria's rebels Ankara isn't seeking direct intervention. And the alliance is thought to be reluctant to get involved militarily at a time when its main priority is the war in Afghanistan.

"Obviously Turkey can rely on Nato solidarity," Rasmussen said before a meeting of Nato defence ministers in Brussels. "We have all necessary plans in place to protect and defend Turkey if necessary."

When pressed on what kind of trouble on the border would trigger action, Nato's chief said he could not discuss contingency plans..

Nato officials said the plans have been around for decades and were not drawn up in response to the Syria crisis.

In an address to lawmakers from the ruling party, Turkish prime minister Recep Tayyip Erdogan reiterated that Ankara will continue retaliating to attacks from Syrian president Bashar al-Assad's regime.

"Every kind of threat to the Turkish territory and the Turkish people will find us standing against it," Erdogan said. "Soldiers loyal to Assad fired shells at us, we immediately reacted and responded with double force. We shall never stop responding."

At least 25 additional F-16 fighter jets were deployed at Turkey's Diyarbakir air base in the south-east of the country late on Monday, Turkey's Dogan news agency said, quoting unidentified military sources. The military's chief of staff inspected troops along the border with Syria on Tuesday.

A Sunni extremist group called Jabhat al-Nusra claimed responsibility for an attack on a Syrian air force intelligence compound in the Damascus suburb of Harasta on Monday evening. A statement on a militant website by the group's media arm, Al-Manara al-Bayda, said the bombing aimed "to avenge the killing of Muslims and those who suffered injustice".

The Syrian state-run news agency did not report the explosion and there were conflicting reports on how badly the compound was damaged. There were no official reports on casualties, but the pro-government al-Ikhbariya channel said the blast was heard across Damascus.

Turkey's state-run Anadolu Agency reported that Syrian National Council leader Abdulbaset Sieda visited rebel-controlled areas in Syria on Tuesday.

It said he entered Syria from the Bab al-Hawa border crossing and "made observations in rural areas" of Idlib province before travelling to Aleppo's Etarib area, where he met with commanders of the Free Syrian Army. If confirmed, the trip would be Sieda's first into Syria since he became the council's leader in June.

Anadolu quoted Sieda as saying: "We are here to see what the opposition in Syria and the opposition outside of Syria can do together to serve the Syrian people."