NATO warned Tuesday against any escalation in hostilities between Syria and Turkey after border artillery exchanges as alliance defence ministers prepare for a difficult withdrawal from Afganistan.

"I would like to commend the Turkish government for the restraint it has shown in its response to the completely unacceptable Syrian attacks," NATO head Anders Fogh Rasmussen said at the opening of a two-day ministers meeting.

"Obviously Turkey has a right to defend herself within international law," he said.

"We hope it (protecting Turkey) won't be necessary, we hope that both countries will show restraint and avoid an escalation of the crisis," he said.

Turkey as a NATO member has the right to invoke military help in response to an attack on its territory under Article V but it has so far invoked only Article IV, which involves consultations.

Pressed later in the day, Rasmussen reiterated his position, insisting on the importance of a political solution in Syria and saying there was no reason to focus on the issue of NATO contingency plans being ready for Turkey's defence.

"I think you would be surprised if a defence alliance ... didn't have in place necessary plans to defend and protect all our allies," he said.

Rasmussen said in his opening remarks that the pullout of NATO Afghan combat forces by end-2014 was the top concern of the talks that will also review defence commitments and how to meet them in times of austerity.

Despite setbacks, for NATO, "the goal, the strategy and the timeline remain unchanged," he said, as the alliance begins work on a new mission of training and assistance from 2015.

The meeting must also address the tricky problem of military spending at a time when governments are under pressure on all sides to cut budgets.

"This period of economic austerity poses a challenge to defence budgets but it also opens an opportunity for strength and cooperation and new ways to provide security," the secretary general said.

NATO adopted a 'Smart Defence' policy at a May summit in Chicago, aiming to get a 'bigger bang for its buck' through cooperation and burden sharing to offset smaller budgets.

Rasmussen told a press conference after the first day that ministers had reviewed progress since Chicago, with European countries playing a central role in joint projects.

The United States accounts for the bulk of NATO member state defence spending and its portion has increased markedly in the last decade, a sore point for Washington in trying to get its allies to do more.

"Let me be clear -- we need smart spending and even more, we need sufficient spending," Rasmussen said.

He declined comment on the proposed merger of European aerospace giant EADS and British arms maker BAE Systems as a it was a commercial matter but welcomed consolidation in Europe's defence sector.

"In general I am in favour of restructuring the European defence industry, the companies, to make them more effective and competitive."

The deal would create the world's biggest civil and defence aerospace company, more than a match for US giant Boeing, but it had appeared to be in trouble ahead of a Wednesday deadline.

Sources close to the negotiations, however, said France and Britain had agreed that total state holdings in an EADS-BAE entity would be limited to 18 percent. The position of Germany, the last state shareholder involved, was not immediately known.

British Defence Minister Philip Hammond said he hoped to discuss with his French, German and US counterparts the proposed tie-up.

BAE has huge interests in the US defence market and Washington has voiced misgivings the new entity might carry too much European government influence.