"This could seriously complicate our efforts. Not just our efforts but the efforts of the parties to extend their negotiations," US State Department spokeswoman Jen Psaki said after the announcement of a reconciliation agreement between Palestinian Authority President Mahmoud Abbas's Fatah and the Islamist Hamas in Gaza.
"The timing was troubling and we were certainly disappointed in the announcement," she told the daily State Department press briefing.
The Palestinian move envisions forming a unity government within five weeks and holding national elections six months after a vote of confidence by the Palestinian parliament.
The announcement comes as the US deadline for a framework on final status issues on a peace accord between the Israelis and the Palestinians rapidly approaches at the end of the month.
The US designated Hamas as a foreign terrorist organization in 1997. The group has not been a member of U.S.-brokered peace talks.
Hamas is also on the EU list of terror groups.