Cúcuta (Colombia) (AFP) - International pressure mounted Sunday against Venezuela's leader Nicolas Maduro, with Washington insisting his "days are numbered" after opposition efforts to bring humanitarian aid into the country descended into bloody chaos.

Self-declared interim president Juan Guaido called on the international community to consider "all measures to free" Venezuela after clashes at border crossings left four people dead since Friday.

The European Union condemned the use of violence and armed civilian groups to block the entry of aid by Maduro's government, which claimed victory.

United Nations Secretary-General Antonio Guterres said he was "shocked and saddened" by the civilian deaths.

Guaido is set to participate in Monday's Lima Group meeting of mostly Latin American countries in Bogota, and called on the international community to be prepared for "all possibilities" regarding Maduro.

Colombia's President Ivan Duque said Venezuela's "legitimate government" was formally joining the group at the meeting, where US Vice President Mike Pence will represent Washington.

US Secretary of State Mike Pompeo said he was confident that "Maduro's days are numbered," blaming the border violence on armed loyalists known as "colectivos."

"We're aimed at a singular mission -- ensuring the Venezuelan people get the democracy they so richly deserve," he said on CNN's "State of the Union."

President Donald Trump has said that Washington is not ruling out armed action.

- 'Victory consolidated' -

Humanitarian aid, much of it from the United States, has become the centerpiece of the standoff between Maduro and Guaido, the 35-year-old leader of Venezuela's National Assembly who declared himself acting president one month ago.

The country is gripped by a humanitarian crisis that has seen poverty soar during a prolonged recession and hyperinflation.

"Today we consolidated yesterday's victory, tomorrow we'll consolidate it even more," said Maduro's right-hand man, Diosdado Cabello.

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"Not even one humanitarian aid truck got through."

Maduro claims the aid is a smokescreen for a US invasion, and has ordered several crossings on Venezuela's borders with Colombia and Brazil closed.

A 14-year-old boy was among those killed Saturday near the Brazilian border in clashes Saturday with Venezuelan security forces. More than 300 people were injured in a day of disturbances at crossings on the Colombian and Brazilian borders.

"We repudiate the use of irregular armed groups to intimidate civilians and lawmakers who have mobilized to distribute assistance," EU foreign policy chief Federica Mogherini said.

She added that the EU was prepared to "scale up" humanitarian and development aid.

Guterres appealed "for violence to be avoided at any cost."

At the border with Brazil, youths threw stones and other objects against National Guard troops in riot gear on the Venezuelan side of the border who threw tear gas and were backed by anti-riot vehicles.

After the clashes ended, some 500 Maduro supporters -- some dressed in red, the color of the PSUV ruling party -- hoisted the Venezuelan flag that demonstrators on the Brazilian side had lowered the day before.

Guaido had set a Saturday deadline for delivering food and medical aid stockpiled in Colombia and Brazil.

"We didn't give up, nor will we give up," said Cabello.

"We didn't fall into their trap, we didn't give them the dead they wanted."

Hundreds of Venezuelans were frustrated in their attempts to take aid across the border from Colombia, pushed back by Maduro's security forces.

A ship with aid from Puerto Rico was also forced to turn back after receiving a "direct threat of fire" from Venezuela's military, the governor of the US territory Ricardo Rossello said. He slammed the move as "unacceptable and outrageous."

- Violent clashes -

Protesters in the border towns of Urena and San Antonio were kept at bay by the Venezuelan National Guard firing tear gas and rubber bullets.

Gunshots could be heard in the streets of Urena during hours of rioting. Civil defense officials in Colombia said at least 285 people had been injured in clashes at border bridge crossings.

Maduro's supporters also halted and set ablaze two trucks loaded with aid driven through barricades on a border bridge.

The number of Venezuelan military and police defections rose to 156 crossing the border into neighboring Colombia, immigration services said there.

Guaido -- recognized as interim leader by more than 50 countries -- has offered amnesty to all security personnel switching sides.

- Maduro defiant -

Angered by Colombia's support for Guaido, socialist leader Maduro announced that Caracas was severing diplomatic ties with Bogota, and gave Colombian diplomats 24 hours to leave the country.

"I will never bow down, I will never give in. I will always defend our country with my own life if necessary," Maduro told a rally of his supporters in Caracas.

A separate opposition rally saw thousands demonstrate outside La Carlota military airport in Caracas.

As many as 300,000 Venezuelans face death if aid isn't delivered after years of shortages and malnutrition, according to Guaido, who has accused Maduro of rigging his re-election and is demanding a new vote.

UN figures show that 2.7 million people have fled Venezuela since 2015 and around 5,000 Venezuelans emigrate each day.

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