U.S. military officials in Iraq say four American soldiers have been killed in two separate roadside bombings in Baghdad.

One soldier died in the hospital from injuries suffered in a blast that went off at about 6:30 a.m. Sunday, local time. Army officials say three soldiers were killed and another wounded late Saturday by another explosion in the capital.

U.S. officials say foreign fighters and terrorists are behind some of the recent major bombings.

To keep insurgents out of Iraq, U.S. administrator Paul Bremer said on Saturday that coalition forces will close 16 of the 19 crossings on border with Iran. Mr. Bremer says he will also double the number of police along the border to about 16,000. Mr. Bremer's spokesman, Dan Senor, said Iranians entering Iraq by land will have to present a passport and register in a database.

Thousands of Iranian pilgrims have been entering Iraq to visit the Shi'ite holy cities of Najaf and Karbala.

Meanwhile, Washington has sent a senior official to Iraq to urge the Iraqi Governing Council to invite the United Nations back to the country.

Some information for this report provided by Reuters, AFP and AP.