Human rights groups from Venezuela and Colombia on Wednesday called on the United Nations to appoint a special envoy to address the humanitarian crisis caused by clashes between Venezuelan troops and Colombian armed groups near the countries' border.Colombian officials estimate that some 4,000 people have fled Venezuela for Colombia since March 21 following a military offensive in Venezuela's Apure state. Some of those who fled to the Colombian border town of Arauquita accused Venezuelan troops of arbitrary detentions, murders of civilians, as well as looting and burning homes, charges that were denied by Defense Minister Vladimir Padrino."We consider it urgent that the Secretary General of the United Nations designate a special envoy for the border crisis," reads the statement, which is signed by 60 organizations including human rights and humanitarian groups.The border between the two nations has been for years a center of drug trafficking and smuggling.The Information Ministry and the United Nations did not immediately respond to requests for comment."Reactions in Colombia after the (armed forces) operations in Apure have been so absurd, fanciful and schizoid, that they only confirm that the Colombian mafias and drug trafficking routes have suffered a strong blow," Foreign Minister Jorge Arreaza wrote on Twitter on Wednesday.His tweet was in response to comments by Colombian Defense Minister Diego Molano about links between Venezuela's armed forces and rebel groups, in an interview with Colombian television station Noticias RCN.The government of President Nicolas Maduro said last week that two soldiers were killed in the clashes, and has said anti-personnel mines where deployed by the armed groups. Local human rights groups say at least four civilians were killed by a police special forces group sent to the area.Colombian President Ivan Duque has said that dissidents from the demobilized Revolutionary Armed Forces of Colombia (FARC) and the Army of National Liberation (ELN) operate in Venezuelan territory with the support of the Venezuelan government. Maduro denies this.