Fighting between the Syrian army and Kurdish forces has intensified, creating the risk of yet another front opening in the multi-sided war.

Government warplanes bombed Kurdish-held areas of Hasaka, one of two cities in the largely Kurdish-held northeast where the government has maintained enclaves.

The two sides have mostly avoided confrontation during the five-year conflict -- the government has been focusing its efforts against Sunni Arab rebels in the west, and the Kurds were mainly fighting the Islamic State in northern Syria.