By Randall Mikkelsen

WASHINGTON (Reuters) - U.S. and Mexican security officials will strategize next month on fighting the arms trade fueling Mexico's bloody drug war, but a U.S. ban on assault rifles favored by the traffickers will not be on the agenda.

U.S. Attorney General Eric Holder and Homeland Security Janet Napolitano announced plans this week to meet their Mexican counterparts -- Attorney General Eduardo Medina and Interior Minister Fernando Francisco Gomez Mont -- at an arms trafficking conference in Cuernavaca, Mexico in early April.

Napolitano told reporters on Thursday the officials will discuss ways of fighting the huge flow of weapons from the United States into Mexico, where they are used by drug gangs in turf battles that killed 6,000 people last year.

To Mexico's great unease, the widely available U.S. guns are often the merchandise carried back along smuggling routes for illegal drugs coming into the United States.

The Mexican drug violence has ignited fears that it would spill over into the United States, and officials in Washington are drawing up new contingency plans for sending military troops if needed to U.S. areas affected by drug violence.

"The only issue is at what level do we (invoke) in the worst-case scenarios, and we're not at the worst-case scenarios yet," she said.

The plans are expected soon, along with a more-immediate Mexican border security plan to be announced separately, Napolitano said.

She said she did not expect regular military forces to be deployed to any violent scenario. Rather, she said, "the real issue is the National Guard right now."

In February, Holder had said the Obama administration would push for renewing a U.S. ban on semi-automatic assault weapons that expired under former President George W. Bush. He said it would "have a positive impact in Mexico, at a minimum."

Assault weapons are among the smugglers' favorite weapons.

But U.S. gun-owners groups, who represent a powerful voting bloc wary of the Obama administration, reacted in sharp protest to Holder's comments.

This week neither Holder nor Napolitano revealed any intention of offering an assault-weapons ban to their Mexican counterparts.

"I think what we're going to do is try to enforce the laws that we have on the books," Holder told reporters on Wednesday.

Napolitano on Thursday gave a similar answer when asked if an assault-weapons ban would be on the table at Cuernavaca.

"There's lots that you can do on the investigation and prosecution side under the existing laws. The key thing is to improve and to keep improving on our interdiction of the weapons before they even get in there."

Issues that will be discussed include cracking down on "straw purchasers" who buy guns on smugglers' behalf in the United States, and more quickly tracing weapons found in Mexico in an attempt to pinpoint major U.S. sellers.

Improving detection systems at the U.S.-Mexican border and ports will be another significant topic, she said.

(Editing by Anthony Boadle)