A row between neighbours Argentina and Uruguay is threatening to overshadow the Ibero-American summit in Chile.
Uruguay has now closed a border crossing, fearing protests
The long-running dispute erupted anew after Uruguay gave an operating permit to a paper mill despite unresolved environmental objections by Argentina.
Separately, Venezuela's leader is set to brief Colombia's Alvaro Uribe over efforts to broker a prisoner exchange between warring parties in Colombia.
Leaders from Latin America, Portugal, Spain and Andorra are meeting in Chile.
The theme of this year's 22-nation summit, which runs until Saturday, is "social cohesion".
But correspondents say the emphasis on harmony threatens to be overturned by the fresh outbreak of hostilities between former friends Argentina and Uruguay.
Uruguayan President Tabare Vazquez granted a long-awaited start-up permit to the mill on Thursday - hours after giving a conciliatory speech at the summit, which he ended by hugging Argentine President Nestor Kirchner.
The paper mill has soured relations between old friends
On Friday, Uruguay announced it had closed its border crossing with Argentina closest to the mill in Fray Bentos, after Argentine campaigners pledged to protest there on Saturday.
The moves led to protests from the Argentine delegation in the Chilean capital, with Mr Kirchner blaming Mr Vazquez for putting an end to efforts by Spain's King Juan Carlos to mediate a resolution to the dispute.
"You have stabbed the Argentine people in the back," Mr Kirchner told his counterpart according to the official Argentine news agency Telam.
This is the latest instalment of a two-year row.
The Finnish owners of the pulp mill - the biggest foreign investment in Uruguay - insist it employs the latest technology and will not pollute. But Argentina disagrees and has taken the case to the International Court in the Hague, whose ruling is pending.
Also on the sidelines of the summit, Venezuela's President Hugo Chavez is expected to brief Colombian President Uribe on Saturday.
Mr Chavez met a leading member of the left-wing Farc rebel group on Thursday to discuss exchanging Farc's prisoners for rebels held in Colombian prisons.
The Uribe-Chavez amity has surprised many observers
He has been mediating between the Colombian government and Farc to negotiate a prisoner swap. Farc wants 500 rebel prisoners freed in exchange for the release of about 50 high-profile hostages it has been holding in jungle camps for years.
They include French-Colombian citizen and former presidential candidate Ingrid Betancourt, and three US defence contractors.
On Thursday, Mr Chavez said he was "more optimistic than ever over this humanitarian exchange issue".
The intervention of the colourful left-wing leader in the affairs of a close ally of the US has surprised some observers.
Farc formed in the 1960s to overthrow the government but since then has been drawn into kidnapping and the illegal drugs trade.