Guyana says the Venezuelan navy has entered its territorial waters and detained a US-operated ship.
The government said the Venezuelan actions in the disputed Essequibo region constituted "a serious threat to peace in the region".
The ship was chartered by Texas-based company Anadarko, which has been granted a contract to look for oil in the area.
Caracas has said the ship was operating illegally in Venezuelan waters.
The US has declined to comment on the seizure.
The vessel - the Teknik Perdana - was surveying the seafloor on Thursday when it was approached by a Venezuelan navy vessel and forced to sail to Venezuela's Margarita Island. At least five US citizens are on board, the company said.
"One point is clear: the Teknik Perdana was in Guyana's waters when this incident took place," said the Guyanese foreign ministry in a statement.
The US state department said it was aware of reports that five US citizens were among crew members detained by Venezuelan authorities aboard the ship.
"Due to privacy concerns, we cannot comment further at this time," a statement said.
The ship sails under a Panamanian flag and is owned by a Malaysian marine surveying company.
"Our first concern is the safety of the crew of the MV Teknik Perdana research vessel, which was under contract to our company and conducting a seafloor survey on behalf of the government of Guyana," said a spokesperson for Anadarko, Brian Cain.
"We are fully cooperating with the Government of Guyana, the US coast guard and embassy personnel in an effort to achieve the safe release of the crew and vessel," Mr Cain added.
The Venezuelan foreign ministry responded shortly after, issuing a statement demanding an official explanation from the Guyanese authorities.
"Venezuela expresses its deepest concerns over the manner in which foreign vessels authorised by the government of Guyana enter Venezuelan territorial waters and exclusive economic area," said the statement.
"We reiterate that the Bolivarian National Navy would never encroach on the territory of a fraternal nation."
According to the statement, the vessel is expected to dock in the Margarita Island at 06:00 local time (11:30 GMT) on Saturday.
The foreign ministry reaffirmed Venezuela's "peaceful vocation" and said it had no intention of "going back to previous situations of confrontation with the Republic of Guyana".
The vast area west of the Essequibo river makes up two-thirds of the territory of Guyana.
It has been claimed by Venezuela as its own since the 19th Century, when Guyana was still a British colony.
Former Venezuelan President Hugo Chavez made attempts to improve relations between the two South American neighbours.
In an official visit to Georgetown in 2004, he played down the dispute and said that "the integration of South America and the Caribbean is vital, especially now that neo-liberalism has failed".