* Bill has 48 Senate co-sponsors, up from 26 upon introduction
* Foreign policy group says bill could push U.S. closer to war
* White House has threatened to veto bill
By Timothy Gardner
WASHINGTON, Jan 6 (Reuters) - U.S. senators pushing a bill to slap new sanctions on Iran if it goes back on an interim deal under which it agreed to limit its nuclear program have gained support since the legislation was introduced in December, aides said on Monday.
The bill, which the White House has threatened to veto, requires further reductions in Iran's oil exports and would apply new penalties on other industries if Iran either violates the interim agreement or fails to reach a final comprehensive deal.
Iran signed the six-month interim deal in Geneva on Nov. 24 with the United States, Britain, France, Russia, China and Germany.
The "Nuclear Weapon Free Iran Act" had about 48 co-sponsors in the 100-member Senate on Monday, up from 26 when the bill was introduced on Dec. 19, an Senate aide said.
"Expect that number to keep growing over next couple of days as folks who were out of town and staff get back in," the aide said.
The bill was introduced by Robert Menendez, the chairman of the Senate Foreign Relations Committee, and Mark Kirk, a Republican from Illinois.
"We expect several Democrats to kind of cross the picket line and come on board this week," the aide said.
While the bill has gained support, it remains uncertain if backers can put together the two-thirds majority in the Senate needed to override a veto by President Barack Obama.
The Obama administration has insisted the bill would damage delicate talks being held between Iran and world powers over the nuclear program, which Tehran says is for peaceful purposes. Iranian Foreign Minister Mohammad Zarif has said a new sanctions law would kill the interim agreement.
While senior Democrats in the Senate like Menendez, from New Jersey, and Charles Schumer, from New York, support the new sanctions, there is a strong bloc of opposition in the party. Ten Democratic senators, all leaders of committees, sent a letter to Senate Majority Leader Harry Reid last month expressing their opposition to the bill.
A bipartisan group of nine senior foreign policy experts urged Menendez and Kirk not to pass the new sanctions, saying the penalties could potentially move the United States closer to war.
Ryan Crocker, a former ambassador to Iraq and Afghanistan, and Thomas Pickering, former Ambassador to Israel, India and the United Nations, were among signers of a letter to the senators that said a sanctions bill, even if it took effect in six months, would call into question Washington's good faith and possibly isolate the United States among the countries holding talks with Iran.
The bill gives the administration up to a year to pursue a diplomatic track, which backers say would not violate terms of the interim deal.
On oil exports, the new bill seeks to cut sales at least 30 percent within a year and to zero within two years, if Iran breaks the deal or a comprehensive deal is not reached.
Since 2012, U.S. and European sanctions have cut Iran's oil sales by more than 1 million barrels per day, costing the country billions of dollars a month and driving up inflation. (Additional reporting by Patricia Zengerle, editing by Ros Krasny and Cynthia Osterman)