By SIMON ROMERO

CARACAS, Venezuela — Fuming over assertions by American officials that cocaine smuggling through Venezuela has surged in recent years, President Hugo Chávez threatened the American ambassador with expulsion on Sunday, opening a new phase of tension between Venezuela and the United States.

Speaking on his Sunday television program, Mr. Chávez also called the Bush administration’s drug czar, John P. Walters, “stupid,” mocking him by breaking into English and asking, “Are you a donkey?” Going further, Mr. Chávez described the United States as hypocritical, calling it the largest producer of marijuana.

Mr. Chávez was responding to an assertion by Mr. Walters that flows of Colombian cocaine through Venezuela had quadrupled since 2004, reaching about 282 tons in 2007. The American ambassador, Patrick Duddy, later asserted that drug traffickers were taking advantage of tense relations between the United States and Venezuela.

Mr. Chávez’s comments effectively ended what seemed to be the start of a thaw in July, when he chatted with Mr. Duddy at a military parade and invited him to lunch.

“We continue to respond to President Chávez’s overture from July,” Jennifer Rahimi, a spokeswoman for the embassy, said Sunday.

The expulsion threat to Mr. Duddy is the first he has received since arriving here a year ago. Mr. Duddy has kept a lower profile than his predecessor, William R. Brownfield, a loquacious envoy whom Mr. Chávez repeatedly threatened with expulsion.

Seizing on other issues with the United States, Mr. Chávez reiterated an invitation to the Russian Navy to visit Venezuela and said his government would buy a long-range missile system from Russia to complement its Russian-made Sukhoi fighter jets.