WASHINGTON -- New U.S. security rules are in effect banning all cargo from Yemen and Somalia and prohibiting toner and ink cartridges weighing more than one pound (half a kilogram) from passenger flights, Homeland Security Secretary Janet Napolitano said Monday.

The new rules are a direct response to the thwarted terror plot that could have taken down two cargo planes over the U.S. last month. Terrorists in Yemen had hidden two powerful bombs inside printers and shipped them to addresses in Chicago.

As the packages made their way to the U.S., Saudi Arabia tipped off intelligence officials to the plot, providing the FedEx and UPS tracking numbers that allowed officials to pinpoint where the packages were en route.

"The threats of terrorism we face are serious and evolving, and these security measures reflect our commitment to using current intelligence to stay ahead of adversaries," Napolitano said in a statement.

The U.S. immediately banned cargo from Yemen after the bombs were intercepted. Other countries including Britain and Germany -- which the bombs traveled through -- followed suit.

Somalia was added to the U.S. ban, despite a lack of intelligence pointing to a similar plot to detonate bombs on cargo planes, said a senior administration official who spoke on condition of anonymity because he was not authorized to speak publicly. The official said the terrorist group in Somalia, al-Shabaab, has said it intends to attack the U.S., just as al-Qaida in the Arabian Peninsula has stated and tried to do.

Britain is also banning all cargo from Somalia as well as large printer cartridges transported by air.

Besides the bans, high risk cargo will no longer be allowed to fly on passenger planes, Napolitano said, without elaborating what constitutes high risk. Until now, the U.S. has required that high risk cargo be screened before it's loaded onto U.S.-bound passenger planes. Some printer ink and toner cartridges weighing more than a pound shipped internationally will also be banned from flying to the U.S., but Napolitano and the senior administration official did not say which toner and ink shipments would be banned. The senior administration official instead pointed to the ban on all cargo from Somalia and Yemen. The new rules also include extra screening for all high risk cargo, which could include physically opening a package and inspecting it, explosive detection and X-rays, the official said. How a high risk package is screened will be left up to the company or the country, he said.

About 30 percent of air cargo shipped to the U.S. is shipped on passenger planes, according to the Bureau of Transportation Statistics.

Analysts warn that the cost of screening every piece of air cargo in a bid to prevent terrorists from downing airliners might bankrupt international shipping companies, hobble already weakened airlines and still not provide full protection.

On Monday, European Union interior ministers established a panel to review a proposed plan to tighten air cargo security that would include blacklisting high-risk airports that are deemed to have inadequate security measures.