BEIRUT -- NATO is ready to defend alliance member Turkey amid artillery and mortar exchanges with Syria, its top official said Tuesday, as Ankara sent additional fighter jets to reinforce an air base close to the Syria border where tensions have escalated dramatically over the past week.

Turkey and Syria have exchanged fire across their common border since errant Syrian shells killed five Turkish civilians last week, sparking fears of a wider regional crisis.

The comments by NATO Secretary-General Anders Fogh Rasmussen were the strongest show of support to Ankara since the firing began Wednesday -- though the solidarity is largely symbolic. Turkey has sought NATO backing but not direct intervention and the alliance is thought to be reluctant to get involved military at a time when its main priority is the war in Afghanistan.

Ahead of a meeting of NATO defence ministers in Brussels, Fogh Rasmussen backed Turkey's right to defend itself.

"Obviously Turkey can rely on NATO solidarity," he added. "We have all necessary plans in place to protect and defend Turkey if necessary."

"We hope that all parties involved (in the Syrian crisis) will show restraint and avoid an escalation of the crisis," Fogh Rasmussen said.

NATO officials said the plans have been in place for decades and were not drawn up in response to the Syria crisis. They spoke on condition of anonymity because they were not authorized to talk to the press.

In an address to lawmakers from the ruling party, Turkish Prime Minister Recep Tayyip Erdogan reiterated that Ankara will continue retaliating for attacks from the regime of Syrian President Bashar Assad.

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

Analysts say Syria appears to be intentionally escalating tensions along the border with Turkey to send a message to its northern neighbour that it will pay a high price for its support of the Syrian rebels, hoping that will deter any foreign military intervention in the 18-month-old civil war.

At least 25 additional F-16 fighter jets were deployed at Turkey's Diyarbakir air base in the southeast late 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.

The reinforcement of the Diyarbakir base also bolsters Turkish forces along the volatile Iraqi border. Turkish jets struck Kurdish rebel targets in northern Iraq in two separate cross-border raids Sunday despite recent warnings from Baghdad against any military operations on its territory.

Turkey has frequently struck targets in northern Iraq of the Kurdistan Workers' Party, or PKK, which seeks autonomy for Turkey's Kurdish minority. Relations between Turkey and Iraq have also been deteriorating over a Turkish decision to shelter convicted Iraq's Sunni vice-president on charges of running death squads.

Activists estimate more than 32,000 people have been killed since March 2011 when the uprising against Assad's regime began. Initially, regime opponents launched a wave of peaceful protests that were met by repeated attacks by security forces, and the conflict has gradually turned into bloody civil war that has prompted tens of thousands of civilians to flee Syria. The fighting has devastated entire neighbourhoods in Syria's main cities, including the northern city of Aleppo. Syria's government has always blamed the uprising on what it calls foreign terrorists.

A Sunni extremist group called Jabhat al-Nusra claimed responsibility for an attack on Syrian air force intelligence compound in the Damascus suburb of Harasta 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 on Monday the blast was heard across Damascus.

Syrian rebels are increasingly targeting security compounds and symbols of the regime in and around the capital.

Jabhat al-Nusra said it sent two suicide bombers to hit the compound, which it called " a bastion of tyranny and injustice." The compound, headed by Maj. Gen. Jamil Hassan, a top Assad security aide, contains prisons where activists say hundreds of people have been held and interrogated by security services during the uprising.

There were fears that detainees inside could have killed or wounded in the blast, said the Britain-based Syrian Observatory for Human Rights, an activist group. It said that so far it could not confirm casualties from its sources on the ground.

One woman who has a friend currently imprisoned in the compound told The Associated Press she is concerned. "I don't know whether he still alive or he died from the explosion ... I pray to god to save him," she said, speaking on condition of anonymity for fear of harassment from authorities.

On Tuesday, a roadside bomb attached to an electricity pole detonated as a packed commuter bus passed by in the Damascus neighbourhood of al-Zablatani neighbourhood, injuring a civilian, a Syrian official told the AP. The official blamed "terrorists," the regime's term for rebels. He spoke on condition of anonymity because he was not authorized to speak to the media.

Rebels frequently complain that their weapons are no match for the military's artillery and fighter jets. The British Broadcasting Corp. said it had uncovered evidence that appears to suggest that weapons intended for the Saudi military have been diverted to Syrian rebels. Three crates from an arms manufacturer, addressed to Saudi Arabia, were seen by a BBC correspondent in a base being used by rebel fighters in the war-ravaged city of Aleppo.

The BBC said it was not allowed to film their contents.

Meanwhile, two Syrian rebels told The Associated Press that seven military and intelligence officers belonging to Syria's ruling Alawite minority have defected to Jordan. The rebels said they helped the seven cross into Jordan on Monday, and that the highest-ranking figure among them was an army colonel.

Defections by Alawites, who make up the backbone of Assad's regime, are relatively uncommon. Almost all the defections have been from Syria's Sunni majority, who dominate the rebellion.

Three other Alawite intelligence officials came to Jordan three weeks ago, said the two rebels, who spoke to the AP on condition of anonymity for fear of reprisals from the Assad regime. Jordanian officials declined comment.

The new arrivals join more than 3,000 other Syrian army and police defectors who are housed in a secret Jordanian desert installation. The Syrian regime has seen a steady stream of defections in recent months, including former Prime Minister Riad Hijab, who broke with the regime and fled to Jordan in August.