A police spokesman said the separatists stormed their way into a camp of the Central Reserve Police Force in Indian Kashmir's summer capital, Srinagar, on Wednesday.
Nine other people were wounded in the raid on the camp.
The spokesman said intermittent firing between security forces and the separatists continued throughout the night.
Two lesser-known separatist groups, al-Mansoorain and the al-Madina Regiment, claimed responsibility for the attack.
An armed conflict has been raging in Indian-administered Kashmir with a dozen separatist groups seeking independence from New Delhi.
The resultant violence has officially claimed 38,000 lives since 1989. Separatist groups say the toll has been much higher.