Colombian President Alvaro Uribe is to fly to Brazil this week to explain his decision to open three military bases to the United States, an aide to Brazilian President Luiz Inacio Lula da Silva told AFP on Monday.

The visit follows demands from Brazil and other South American nations for the United States to justify the agreement with Colombia.

Several capitals in the region fear it could ignite long-simmering tensions, especially between Colombia and Venezuela.

The Brazilian official said Uribe would travel on Thursday to speak with Lula. The two had a telephone conversation early Monday, he said.

The trip would give Uribe and Lula a chance to talk over several issues, given that Uribe would not attend a meeting of the Union of South American Nations in Ecuador on Monday next week, the official said.

Separately, the US National Security Council said in a statement that US President Barack Obama's national security adviser, retired general Jim Jones, would visit Brazil on Tuesday and Wednesday to discuss "a range of issues of mutual concern."

The topics included "defense relations," the statement said, as well as the global economic crisis and the situation in Honduras.

Colombia, the United States' biggest ally in South America, said last month three of its bases would be used and expanded by the US military as part of Washington's fight against drugs trafficking.

That announcement unnerved Venezuelan President Hugo Chavez, who said he feared the bases could be used for a "Yankee military force" to invade his country.

Venezuela's ties with neighboring Colombia have again entered rocky territory following Bogota's accusations that Chavez supplied Colombian rebels with arms. Chavez has withdrawn his ambassador to Bogota over the claims.

Ecuador, an ally of Venezuela's, is also wary of the bases announcement. It cut off relations with Bogota after Colombia's military staged a cross-border raid into its territory in March last year to destroy a rebel camp.

Both Venezuela and Ecuador nearly went to war with Colombia over the incursion.

Brazilian Foreign Minister Celso Amorim told Sunday's edition of the Folha de S. Paulo newspaper that the US bases in Colombia were of concern.

"What worries Brazil is a strong military presence whose aim and capability seems to go well beyond what might be needed inside Colombia," he said.

He said Brasilia was asking for explanations from Washington.

Lula last week said "I'm not happy with the idea of another US base in Colombia."

Chile and Spain have also expressed unease.