The advance of Islamic State rebels on the Syrian border town of Kobani is a tragedy but will not deter the United States and its allies from their long-term strategy in the region, US Secretary of State John Kerry said on Thursday.

"Kobani is a tragedy because it represents the evil of ISIS but it is not the definition either of the strategy or the full measure of what is happening with response to ISIS," Kerry told reporters in Boston, using an acronym for Islamic State rebels.

Rebels have seized more than a third of Kobani, advancing despite US-led air strikes as Turkish troops looked on but did not engage.

"We are only a few weeks into building the coalition. People are still receiving assignments," Kerry said. "The primary goal of this effort has been to provide the space for Iraq to be able to get its government in place and to be begun to push back."