Dozens of Palestinian gunmen, including off-duty policemen, blocked two major roads in the Gaza Strip for more than an hour Saturday in the latest sign of chaos gripping the area ahead of Jan. 25 elections.

With the violence bolstering Hamas' prospects in the legislative election, U.S. officials warned Saturday that millions of dollars of aid could be in jeopardy if the militant Islamic group joins the Palestinian government. Palestinian leader Mahmoud Abbas said Hamas' participation in the government was "not an American issue."

The gunmen were demanding a government response to the killing of a policeman in a drug bust a week ago.

The protesters urged Abbas to impose order in Gaza. They also called for the resignation of Interior Minister Nasser Yousef, who oversees Palestinian security.

The plainclothes officers said they are members of the Al Aqsa Martyrs' Brigades, a militant group linked to Fatah.

On Sunday, Israel closed the main cargo terminal between Israel and the Gaza strip until further notice, citing intelligence warnings of a planned Palestinian attack as the reason for shutting the Karni crossing, army radio said.

Opinion polls show Hamas in a close race with Fatah. Hamas is committed to Israel's destruction and has been labeled a terrorist group by the United States and European Union.

A U.S. official said Saturday that Washington would have to review its aid to the Palestinians if Hamas joins their government.

Abbas, marking one anniversary in power on Saturday, told the Al-Jazeera network that Hamas' possible participation in the Palestinian government "is not an American issue."

"If Hamas wants to participate in the Palestinian Authority, we have no objection to that," Abbas said.

Under the interim peace accords that the Palestinians and Israel signed in Oslo, Norway, in the early 1990s, groups that participate in the Palestinian government must renounce violence.

Ismail Haniye, who tops Hamas' parliamentary list, said in a statement that he "does not rule out Hamas' participating in the government or forming a government itself if it gets a majority in parliament."

Hamas' election platform has focused primarily on internal Palestinian issues, like cleaning up government corruption and restoring law and order, though it also refers to the right to "resistance" against Israel.

In the West Bank city of Hebron, crowds of Jewish settlers threw stones at Palestinian homes and tried to force their way into a market there before Israeli security forces restrained them, security forces said.

Tensions have been high in Hebron since Israel issued orders to evacuate eight settler families from a market area they took over four years ago after Palestinian gunmen killed a 10-month-old Israeli baby.