Maduro asks armed forced to to be 'vigilant' to deter US invasion.

Venezuelan President Nicolas Maduro on Wednesday ordered armed forces to be "vigilant" to deter a possible United States invasion.

"I have ordered all military branches to be vigilant and be ready to protect the peace, to make any attempt by the North American empire to try anything against our homeland impossible. Peace will be our victory!" Maduro tweeted in Spanish.

This comes after Guaido's envoy to Washington Carlos Vecchio has sent a letter to US Southern Command calling for "strategic and operational planning so that we may fulfil our constitutional obligation to the Venezuelan people", reported Sputnik.

Venezuela is witnessing a political crisis, which has exacerbated after National Assembly leader Juan Guaido declared himself to be the President of the nation, calling for the elected Maduro, to step down.

The United States immediately extended support to Guaido, recognising him as the official interim President. Maduro, however, has refused to resign, citing the support of the armed forces.

Maduro has accused the US of trying to orchestrate a coup in order to install Guaido as its puppet and take over Venezuela's natural resources.

Washington also slapped fresh sanctions on two shipping firms for allegedly transporting Venezuelan oil to Cuba.

In a statement, the US State Department said on Friday, "We have designated two companies, Monsoon Navigation Corporation and Serenity Maritime Limited, that operate in the oil sector."

The US has been pursuing a policy of economic sanctions and diplomatic isolation against the Maduro government in support of Guaido.