Hamas has accused forces loyal to the Palestinian President of damaging reconciliation efforts after a raid on Hamas forces in the West Bank left six dead.
Three policemen, two members of Hamas and another man were killed in a gun battle when police carried out a dawn raid on Hamas in Qalqilya.
Officials said the police were loyal to Palestinian leader Mahmoud Abbas, of Fatah.
The BBC reports it was one of the worst clashes in the West Bank since Hamas seized control in the Gaza Strip in 2007.
Hamas has been in charge of the Gaza Strip for nearly two years. Fatah controls the West Bank.
Both sides have been accused by human rights groups of abuses of power against rival supporters during that time.
The BBC says the two factions are supposed to be in a reconciliation process to end a deep split.
Solving Fatah-Hamas differences is seen as an essential step if an Israeli-Palestinian peace deal is to happen.
Unlike the PLO which is dominated by Fatah, Hamas refuses to recognise Israel's right to exist and has waged a violent campaign against it for years.