HOUSTON — The Trump administration moved Thursday to protect the Venezuelan-owned refining company Citgo from creditors by closing a narrow loophole on sanctions intended to topple the Venezuelan government.

Citgo faced dismemberment if the Venezuelan national oil company did not make a $913 million payment on its 2020 bonds due on Monday. But now all transactions on the bonds will be blocked for 90 days.

The administration move will shore up Juan Guaidó, the Venezuelan opposition leader. Mr. Guaidó declared himself the acting president after elections last year that were widely considered fraudulent but kept President Nicolás Maduro in power.

In recent months Mr. Guaidó managed to take control of the board of Citgo, which is an affiliate of Petróleos de Venezuela, or PDVSA. But his position in Venezuela has weakened after several failed attempts to prompt a military uprising against Mr. Maduro, who has the support of Cuba and Russia.