US-Venezuelan relations are said to have hit a new low
A series of tit-for-tat expulsions has left the US without ambassadors in three Latin American countries.
Bolivia and Venezuela have expelled their US envoys, accusing Washington of trying to oust Bolivia's government.
Washington has responded by throwing out envoys from Bolivia and Venezuela and freezing the assets of three aides to Venezuelan President Hugo Chavez.
In Bolivia, where the dispute began, President Evo Morales declared a state of emergency in a remote jungle region.
Mr Morales imposed the measure on the northern province of Pando, which borders Brazil, even as he prepared for talks with the leader of another opposition-controlled region.
Bolivian troops were reported to have taken control of the airport in the city of Cobija, capital of Pando, with anyone carrying weapons in the city threatened with arrest.
In Washington, US State Department spokesman Sean McCormack said the US regretted the actions of Venezuela and Bolivia.
"This reflects the weakness and desperation of these leaders as they face internal challenges, and an inability to communicate effectively internationally in order to build international support," he said.
Bolivian and Venezuelan allegations - including that the US supports continuing anti-government protests in Bolivia - were false "and the leaders of those countries know it", Mr McCormack added.
Meanwhile, Honduras has refused the credentials of a new US ambassador, postponing his appointment.
The BBC's Emilio San Pedro said relations between the US and Latin American opponents such as Mr Chavez had seemed to be on a holding pattern - but the situation has changed in a matter of days.
This week's arrival in Venezuela of two Russian bomber planes taking part in a military exercise is not thought to have helped the situation.
And with more joint military exercises in the pipeline, our correspondent says it could take a while for tensions to subside.
Freezing the assets of the three Venezuelan aides, the US Treasury accused them of "materially assisting the narcotics trafficking" of rebels in Colombia.
All three had "armed, abetted and funded the Farc, even as it terrorised and kidnapped innocents", according to a statement from the US Treasury referring to the left-wing rebel Revolutionary Armed Forces of Colombia (Farc).
Analysts say the trio - Hugo Armando Carvajal Barrios, Henry de Jesus Rangel Silva and Ramon Rodriguez Chacin - are members of Mr Chavez's inner circle.
Evo Morales accused the US envoy of meddling in Bolivia's internal affairs
Mr Carvajal Barrios is a military intelligence director who has protected Farc drug shipments from seizure, claimed the US statement.
Mr Rangel Silva is another intelligence chief who had pushed for greater co-operation between Venezuela and the Farc, the US Treasury alleged.
And Mr Rodriguez Chacin, who until Monday was Venezuela's justice minister, is Caracas' main "weapons contact" for the Farc, the statement charged.
The flurry of diplomatic expulsions began on Thursday, when Bolivia threw out the American ambassador to La Paz, Philip Goldberg.
President Evo Morales said the US envoy had been siding with a violent opposition movement in the east of Bolivia, where groups are demanding greater autonomy and a bigger share of gas export revenues.
'Go to hell'
US officials said the allegations were baseless, but nonetheless expelled the Bolivian ambassador to Washington in retaliation.
This prompted the Venezuelan leader to step into the fray alongside his Bolivian ally.
President Chavez gave US ambassador Patrick Duddy 72 hours to leave Caracas, telling him: "Go to hell 100 times."
On Friday, Washington responded by giving the Venezuelan ambassador his marching orders.
Now Honduran President Manuel Zelaya has refused to accept the credentials of a new US ambassador.
"We are not breaking relations with the United States. We only are [doing this] is solidarity with Morales, who has denounced the meddling of the United States in Bolivia's internal affairs," Mr Zelaya said.
In a separate development, Nicaragua's President Daniel Ortega said he supports Bolivia, but did not announce whether he would take any action against the US envoy in Managua.
BBC South America correspondent Daniel Schweimler says a growing number of left-wing Latin American governments have backed Mr Chavez's anti-US rhetoric.
The region has also benefited from the Venezuelan leader's generosity with oil.
But the US is a leading trade partner and a major aid donor to Latin America, so few in the region will be happy relations have plummeted to this new low, according to our correspondent.
He says this diplomatic row is serious but will probably soon blow over, while Bolivia's problems are only likely to get worse.