MADRID, April 3 (UPI) -- A Muslim extremist group has threatened Spain with more terrorist attacks if it does not withdraw all its troops from Iraq by May.

Several Spanish newspapers Saturday reported the Spanish embassy in Egypt had recently received a letter signed by the Abu Hafs al-Masri Brigades threatening to attack Spanish embassies and Spanish interests in north Africa and the southern and eastern Mediterranean region.

The letter warned the attacks would go ahead unless Spain withdrew its troops from Iraq and Afghanistan within four weeks, El Mundo reported.

Meanwhile, Spanish soldiers stepped up patrols along rail links across Spain as part of a package of security measures introduced after the discovery of an unexploded bomb on Friday.

Police found the device on a high-speed railway between Madrid and Seville. The explosive content is said to be the same as that used in last month's Madrid train bombs, which killed 191.

Forty-five helicopters, sniffer dog teams and armored vehicles are involved in the security operation.