By Patricia Zengerle

WASHINGTON (Reuters) - Two of President Donald Trump's fellow Republicans joined four Democrats on Monday in demanding that the White House provide more information about an executive order that has sown confusion among international organizations involved in family planning, AIDS treatment and other healthcare issues.

In one of his first actions as president, Trump signed an executive order on Jan. 23 reinstating the so-called Mexico City policy, known by critics as the "global gag" rule, which withholds U.S. funding for international organizations that perform abortions or provide information about abortion.

Although other Republican presidents have also adopted the policy, Trump broadened the scope to all global health assistance, "which may encompass as much as fifteen times more federal funding than previous Republican administrations' versions of this policy," the six senators wrote in the letter, which was seen by Reuters.

The order withholds half a billion dollars or more in U.S. funds, and aid groups said it was issued with so little guidance that they have been scrambling to figure out how to proceed.

In the letter, the senators said Trump's broader order now includes the President's Emergency Plan for AIDS Relief (PEPFAR) and all other global health programs funded through the Department of State, Agency for International Development and Department of Health and Human Services.

PEPFAR, which enjoys broad bipartisan support in Congress, is the largest provider of AIDS-fighting medicine in the world, and has been credited with saving millions of lives.

"This directive ... has caused mass confusion among federal agencies and international relief organizations. While they wait for clarity from this administration, there's been a global chilling effect on life-saving work," said Democratic Senator Jeanne Shaheen, who organized the letter.

Among other things, the senators asked if Trump's administration had conducted a cost-benefit assessment of the policy, whether it had determined how many lives might be saved or lost or whether it had researched how transmission of diseases such as HIV/AIDS and Zika might be affected.

Besides Shaheen, the letter was signed by Republican Senators Susan Collins and Lisa Murkowski, Democrats Ben Cardin and Richard Blumenthal and Angus King, an independent who caucuses with Democrats.

The Trump administration is planning to submit a budget proposing steep cuts in spending on the U.S. State Department and foreign aid. Several Republican members of Congress have expressed reservations about that plan.

(Reporting by Patricia Zengerle; Editing by Jonathan Oatis)