WASHINGTON - An immigration bill endorsed by President Barack Obama easily cleared an important test on Monday when the US Senate backed new border security steps seen as essential to the legislation's fate.

By a vote of 67-27, the border security amendment cleared a procedural hurdle, leaving opponents of the bill with few remaining opportunities for killing or further delaying passage of the legislation this week.

In a sign of the bill's growing strength, 15 Republicans voted with Democrats, who control the Senate.

The bipartisan legislation would bring the biggest changes to US immigration law since 1986, granting legal status to millions of undocumented foreigners who also would be put on a 13-year path to citizenship.

Last week, a small group of senators reached a deal on strengthening border security requirements of the bill by authorizing the hiring of 20,000 more law enforcement agents over the next 10 years and buying high-tech equipment to help stop illegal crossings at the US border with Mexico.

The added security is estimated to cost $46 billion.

The amendment, which is likely to be approved later this week now that the procedural obstacle has been swept away, also calls for finishing construction of 700 miles (1,120 km) of border fence.

The steps were designed to attract more support for the bill from Republicans, who have been concerned that a "pathway to citizenship" for 11 million illegal immigrants would spark a new wave of unauthorized border crossings.

Republican Senator John Hoeven of North Dakota acknowledged that some members of his party in the House of Representatives have called for a more incremental approach to immigration reform than the Senate's comprehensive bill offered.

But Hoeven said, "We have tried to come up with something that is bipartisan so that it can move in the House. Hopefully it (the amendment) will encourage them to move forward."

Hoeven helped write the border security amendment that could propel the immigration bill to a large bipartisan victory in the Senate later this week.

Several conservative Republican senators were not persuaded, however. "This bill has no teeth...it throws $46 billion against the wall" with no guarantees of success, said Senator Tom Coburn of Oklahoma.

Meanwhile, some immigration advocacy groups also expressed frustration with changes to the bill. Frank Sharry, executive director of America's Voice, said of the proposed surge in border patrol agents and other security steps, "We think that what is contained in the amendment is terrible public policy. On the other hand, we recognize that as obnoxious as the amendment is from a policy perspective, it was probably necessary to pass the bill."

Sharry also complained that the amendment prohibits undocumented immigrants from getting credit for Social Security contributions they made while they were working in the country without authorization.

The overall bill, which would give new hope to the 11 million living in the United States illegally and invest more in border security, also would overhaul the US visa system to give American farmers and high-tech firms better access to foreign labor.

If it passes the Senate, as expected, the debate would then move to the House of Representatives, where many Republicans are firmly opposed to granting citizenship to those who have been living in the United States illegally - even if they pay back taxes, learn English and have not been convicted of serious crimes.