US Secretary of State John Kerry on Wednesday said NATO member states wanted to see a clearer defence agreement with Gulf Arab states to fight terror, ahead of summit with Gulf leaders to be hosted by President Barack Obama.

"I think all of the member states feel very strongly that defining a... clearer defence arrangement between the GCC (Gulf Cooperation Council) and other friendly countries and the United States is going to be critical to helping them push back against terrorism," he said at a meeting of NATO foreign ministers in Turkey.

Such an arrangement "would also help fight against some other activities that take place in that region that are unsettling to all of those countries," he added, without specifying further.

Kerry's statement appeared to indicate support for a more formal defence accord between NATO and Gulf states to fight terror, at a time of unusual tensions between some Arabian peninsula states and the US.

Obama had invited six Gulf kings, emirs and sultans to the presidential retreat at Camp David for the summit, seeking to shore up wavering trust while Washington negotiates with regional power Tehran.

But only two heads of state are slated to attend the meeting, with King Salman of Saudi Arabia the most notable absentee.

The meeting comes as a Saudi-set humanitarian truce takes effect in Yemen, after intensive bombing by a military coalition led by the kingdom targeting Iran-backed rebels.