Mr Chavez has denounced Colombia's planned accord with the US
Venezuelan President Hugo Chavez has accused Colombia of carrying out a military incursion into Venezuela.
Mr Chavez said Colombian soldiers had recently been seen crossing the Orinoco river, which forms part of the border, and entering Venezuelan territory.
He said the incursion - which Colombia denies happened - was a "provocation".
South American leaders are gathering in Ecuador for a summit which is set to discuss Colombia's planned accord to allow the US use of its military bases.
Mr Chavez has been embroiled in a diplomatic row with his Colombian counterpart, Alvaro Uribe, since news of the plan emerged.
During his weekly TV show on Sunday, Mr Chavez ordered his troops on to a war footing along the border with Colombia.
"The threat against us is growing," he said. "I call on the people and the armed forces - let's go, ready for combat!"
He said Colombian soldiers had "crossed the Orinoco river in a boat and entered Venezuelan territory", but when Venezuelan troops arrived, they had gone.
"This is a provocation by the government of Uribe," he said. "The Yankees have started to command Colombian military forces."
Venezuela's foreign ministry would file a formal complaint, he added, warning that its military would "respond if there's an attack".
The Colombian foreign ministry said it had been in contact with its military commanders in the border area, who said there had been no such incursion.
Mr Chavez, who is now in Ecuador for the inauguration of President Rafael Correa and a summit of the Union of South American Nations (Unasur), is expected to urge his allies in the region to press Mr Uribe to reconsider the planned accord with the US.
15 Jul: Colombia says deal close to make Colombia regional hub for anti-drug operations
28 Jul: Chavez freezes ties with Colombia, recalls ambassador amid row over base plan, and after Colombia says Farc rebels had Venezuelan weapons
4 Aug: Uribe tours South America to explain planned US accord
5 Aug: Chavez announces trade measures against Colombia
7 Aug: Chavez says he is sending ambassador back to Colombia
9 Aug: Chavez denounces Colombian 'incursion'. Colombia says commandos in Vichada and Guania departments near the border deny any incursion
Ecuador, which has no diplomatic ties with Colombia, and Bolivia have also attacked the plan. Other countries in the region, including Brazil, have sought guarantees that US-Colombian military operations will not spill over Colombia's borders.
The US is leaving its previous regional hub, the Manta air base in Ecuador, after Mr Correa refused to renew the lease.
The deal with Colombia would give the US, which already has forces in the country as part of the anti-drugs programme Plan Colombia, access to air bases in Colombia to gather intelligence and support operations against drugs production and terrorism.
Mr Uribe has said the accord will not infringe Colombia's sovereignty and that there would be no more than 1,400 troops and civilian contractors based there, the maximum permitted under the current military accord between Colombia and the US.
Correspondents say this is not the first time tensions have risen between the Venezuelan and Colombian presidents.
Last year, a war of words culminated in the Venezuelans despatching tanks and heavy armour to the border.