Gates said he does not foresee an expanded policing role for the United States, however. The United States is chiefly involved in distributing relief and will support the United Nations in providing security, Gates said.

"I haven't heard of us playing a policing role at any point," Gates told reporters traveling with him to India for talks on defense cooperation, trade and relations between India and Pakistan.

Gates gave a fuller explanation of the rules under which U.S. troops are operating than the Pentagon previously provided. Gates approved rules of engagement for U.S. forces on shore and aboard nearby ships.

"Anywhere we deploy our troops they have the authority and the right to defend themselves," Gates said, adding that the troops could also defend others "if they see something happen."

On Sunday, Lt. Gen. Ken Keen of the U.S. Southern Command said some violence has hindered rescue workers trying to help earthquake victims. He said providing humanitarian aid requires a safe and secure environment, and while streets have been mostly calm, violence has been increasing.

"We are going to have to address the situation of security," Keen said.

Keen said about 1,000 U.S. troops are in Haiti and 3,000 more are working from ships. More than 12,000 U.S. forces were expected to be in the region by Monday.

Fear of looters and robbers has been among factors slowing aid delivery. After last Tuesday's 7.0 magnitude earthquake, maintaining law and order fell to the 9,000 U.N. peacekeepers and international police already in Haiti, even though those forces also sustained heavy losses in the disaster.

International aid agencies and U.S. officials say they are watching for signs violence is rising as people compete for scarce resources.

"After all we can't deliver the food and water if we don't have a reasonable security situation, so that obviously has to be an element of any work that we're doing with the government of Haiti and the U.N.," Gates said.

A joint statement Saturday from the Haitian president and U.S. Secretary of State Hillary Clinton referred to an expanded U.S. security role.

"President Preval, on behalf of the Government and people of Haiti, welcomes as essential the efforts in Haiti by the government and people of the United States to support the immediate recovery, stability and long-term rebuilding of Haiti and requests the United States to assist as needed in augmenting security in support of the government and people of Haiti and the United Nations, international partners and organizations on the ground," the document reads.

President Barack Obama issued an order allowing selected members of the military's reserves to be called up to support operations in Haiti. Signed Saturday, it lets the Pentagon and Homeland Security Department tap reserve medical personnel and a Coast Guard unit that will help provide port security. More than 250 medical personnel from the Health and Human Services Department are already in Haiti.

The U.S. ambassador to Haiti said Monday that American officials are concerned about security but consider the situation manageable.

"The Haitian police, due to their own significant losses, are degraded," Kenneth Merten said in a nationally broadcast interview. "The U.N. has had losses."

But Merten also said "things are going reasonably well. This is not a perfect law and order situation here even in the best of times."

Merten called the U.S. military presence in and around the island a backup option to handle violence, saying first call would be the Haitian police force and the U.N. force in Haiti. He credited Brazilians in that force with contributing to stability.

"Our troops are standing by in cases where neither the Haitian police nor the U.N. troops are providing security," Merten said on NBC's "Today" show. "In most cases, the Haitian police and the U.N. forces have been able to handle the situation."

Roughly 200,000 people may have been killed in the magnitude-7.0 quake, the European Union said, quoting Haitian officials who also said about 70,000 bodies have been recovered.

Merten said it was too soon to put a number on the economic loss. He described downtown Port-au-Prince, the capital city, as resembling "Tokyo at the end of the Second World War."