US Secretary of State John Kerry has said that drone strikes "may well be" an option to halt the Islamic State of Iraq and the Levant (Isis) insurgency in Iraq.

He added that Washington is "open to discussions" with Iran over the ongoing Iraq crisis and would not rule out military cooperation.

In an interview with Yahoo! News, Kerry said that he would "not rule out anything that would be constructive" in terms of military cooperation with Tehran.

He added that any negotiations or cooperation with Iran would be taken "step-by-step". A US official also revealed that a senior American diplomat may use nuclear talks in Vienna as a setting to discuss Iraq with the Iranian delegation.

US transport vessel USS Mesa Verde is heading to the Gulf with 550 marines on board. It is believed that the ship has been deployed in the event that President Barack Obama decides to conduct military operations in the region.

Kerry's comments come as Isis continue to edge nearer to the Iraqi capital of Baghdad and capture towns in the country's north.

The Sunni militants seized the northern city of Tal Afar, a city which lies between the captured city of Mosul and the border with Syria.