By Richard Cowan and Rachelle Younglai

WASHINGTON (Reuters) - Lawmakers in the U.S. House of Representatives trying to write an immigration bill met on Thursday in a last-ditch attempt to resolve their differences over a temporary worker program and healthcare benefits for illegal immigrants.

The group of eight Democratic and Republican lawmakers has been attempting to craft a bill to overhaul the immigration system and deal with the millions of immigrants living illegally in the United States.

But disputes over border security, work visa numbers and healthcare provisions have risen to the point that there are fears some lawmakers might be on the verge of dropping out.

"I am concerned that the bipartisan group has been unable to wrap up their work," House Speaker John Boehner told reporters, adding, "I know there are some very difficult issues that have come up."

Lawmakers have been arguing over the "triggers" that would define when additional border security steps under the legislation would be sufficient to start legalizing some of the 11 million unauthorized foreigners, sources said.

There also continued to be disagreements over several other policy issues central to an immigration bill, including the number of foreign high-tech workers who would be allowed in, as well as low-skilled construction and service industry employees.

Congressional aides were describing the meeting as "one last-ditch effort" to stay together and produce at least an outline of a bill following about four years of private talks.

All of the House's issues were negotiated in a carefully crafted bipartisan bill now being debated in the Senate Judiciary Committee.

That panel is also struggling with the work visa program in the bill and is under intense pressure from technology companies to make it easier to hire foreign workers.

The committee hopes to complete work on the nearly 900-page bill by the end of this month, clearing the way for debate in the full Senate as early as June.

One of the members of the House group, Republican John Carter, repeatedly told reporters that there was no way the Senate bill would pass the Republican-controlled House.

Immediately following the November 6 elections, in which Hispanic voters roundly rejected Republican presidential candidate Mitt Romney, Boehner called on his party to pivot on immigration.

After years of blocking moves to put the 11 million on a pathway to citizenship that many conservatives call "amnesty," Boehner, the top elected U.S. Republican, urged his party to work for a major revamp of immigration laws.

While citing concerns on Thursday about the lack of progress in the House so far, Boehner said: "I continue to believe that the House ... needs to work its will. How we get there, we're still dealing with it."

STRATEGY DEBATE

The chairman of the House Judiciary Committee, Bob Goodlatte of Virginia, has expressed his preference for doing individual bills tackling parts of the immigration policy problems, instead of a comprehensive approach.

But Democrats in both chambers have rejected such a strategy, saying it would indefinitely delay the pathway to citizenship for undocumented residents, many of whom have been in the United States for decades and are raising families here.

Besides policy disagreements, individual members of the House group have differed over whether it would be productive to unveil a House measure while the Senate is in the midst of debating its bill.

Democrats, for example, have been hesitant to embrace a more conservative House immigration bill, which they fear would undercut their fellow Democrats in the Senate.

(Additional reporting by Caren Bohan; Editing by Fred Barbash, Cynthia Osterman and Eric Walsh)