By Marty Graham

SAN DIEGO (Reuters) - U.S. Homeland Security Secretary Janet Napolitano said on Monday the federal government has made great strides in securing the border with Mexico, as the Obama administration seeks to build momentum this week for immigration reforms.

Conservative officials and lawmakers have accused the government of failing to stem the tide of illegal immigration, and arrests of illegal border crossers went up last year.

"What we've seen now compared to 20 years ago is the difference between a rocket ship and a horse and buggy," Napolitano told reporters after taking a helicopter tour of the San Diego sector of the U.S. border with Mexico.

"I believe the border is secure, the border is safe," she said. "That's not to say it's 100 percent secure - it will never be 100 percent."

In the past 20 years, the government has added thousands of Border Patrol agents, built hundreds of miles of extra security fencing, and added hi-tech surveillance including unmanned surveillance aircraft.

Napolitano said a decline in arrests of illegal border crossers in the patrol's San Diego sector - the westernmost of nine sectors on the nearly 2,000-mile border - demonstrated the government's efforts are paying off.

Agents patrolling the 60-mile strip, which had once been a major thoroughfare for illegal immigrants crossing north from Mexico, made 28,400 arrests in 2012 down from 565,581 in 1992, according to official figures.

Napolitano two years ago used then falling border arrest numbers as a metric of tightening security when she visited the border town of Nogales, Arizona, to set out President Barack Obama's plans to curb Mexico drug smuggling.

But arrests across the entire border with Mexico rose to 356,873 in the year to September 2012 from 327,577 the previous year.

Some conservative Republicans argue that the government has not done enough to boost security. Among critics is Arizona Governor Jan Brewer, who signed a state crackdown on illegal immigrants in 2010.

(Additinoal reporting by Tim Gaynor; Editing by Alex Dobuzinskis, John Stonestreet)