WASHINGTON -- More than 154 kilograms of weapons-grade uranium was transported secretly over thousands of kilometers by truck, rail and ship on a monthlong trip from a research reactor in Budapest, Hungary, to a facility in Russia so it could be more closely protected against possible theft, U.S. officials revealed Wednesday.

The shipment, conducted under tight secrecy and security, included a three-week trip by cargo ship through the Mediterranean Sea, up the English Channel and the North Sea to Russia's Arctic seaport of Murmansk, the only port cleared by Russia for handling nuclear material.

The 13 radiation-proof casks arrived by rail at the secure nuclear material facility at Mayak in Siberia on Wednesday, said Kenneth Baker, an official at the U.S. National Nuclear Security Administration, who oversaw the project.

It is the largest recovery to date of highly enriched uranium provided either by the former Soviet Union or the United States under a program, begun in the 1950s, aimed at spreading the peaceful use of nuclear energy. The two countries have been working to return the spent fuel from reactors around the world because at many of the facilities, including the one in Budapest, security is lax, which raises the possibility of the material being stolen by terrorists.

"It was a big shipment, the biggest one we've ever done," said Baker in an interview hours after he received word that the shipment had arrived at its final destination in Russia. "It was basically enough to make six nuclear weapons."

Under the U.S.-Russian program, the NNSA, part of the Energy Department, has completed 15 recoveries of highly enriched uranium of U.S. origin from research reactors in more than a dozen countries since 2005. The agency also was involved in three earlier shipments of highly enriched uranium of Russian origin that were removed from the Czech Republic, Latvia and Bulgaria and returned to Russia.

But the project targeting the highly radioactive fuel from the Budapest research reactor was particularly complex and challenging, said Baker, the NNSA's assistant deputy administrator for defense nuclear nonproliferation.

It began at 3 a.m. in Budapest in late September and ended Wednesday at the nuclear facility at Mayak. In between, the shipment moved without notice aboard truck and rail to the port of Koper in Slovenia and then by special cargo ship through the ocean shipping lanes that encircle Europe, always staying in international waters at least 20 kilometers from shore, Baker said.

The unusual roundabout route was needed because "we couldn't ship it through Ukraine," which would have allowed a more direct route to Russia, Baker said. The 13 casks were secretly loaded onto trucks at the Budapest research reactor and taken to the city's train station, where it was transported -- one cask per train car -- onto a special train for an eight-hour trip to the port of Koper in Slovenia on the Adriatic Sea.

The shipments then moved through the Mediterranean and the strait of Gibraltar, up the Atlantic and into the English Channel, across the North and Norwegian seas into the icy waters of the Arctic, with a final destination of Murmansk, where it arrived last Saturday. From there, the shipment was loaded on a train for the long trip to Siberia.

"It was the most complicated trip we've ever taken by far," said Baker, who oversaw the loading and early part of the shipment, but did not accompany the shipments after they went to sea, instead returning to Washington.

On Wednesday, he received notice that the shipment had arrived at Mayak, where security is far tighter than it was in Budapest. In Budapest, "they had a fence and a guard," said Baker, although some security improvements have been made with U.S. help over the past year. Still, "you don't want to leave it there, " said Baker.

The Hungarian reactor now is being converted to use low-enriched uranium that cannot be used in a weapon and, therefore, will not be sought by terrorists.

So far, including the shipment from Budapest, 764.3 kilograms of Russian-origin uranium has been retrieved from 11 countries.