Russia is concerned that US military cargo planes may be delivering more than just humanitarian aid to Georgia, a senior Russian military commander said Thursday.

"Let us ask the Americans to convince us that the cargo on board these planes is indeed humanitarian aid," the deputy head of Russia's General Staff Anatoly Nogovitsyn said in a televised press conference.

"Why don't they lift the curtain on what is being delivered? This concerns us fundamentally," he said.

A second US military cargo plane carrying humanitarian aid for conflict-torn Georgia arrived in the capital Tbilisi on Thursday, a State Department official said.

Over the past few days close to two million dollars (1.3 million euros) in humanitarian aid has been provided by the United States to the Georgian people in the form of medical supplies, tents, blankets and water, the official said.

US President George W. Bush announced the start of the aid missions Wednesday, warning Russia it had to ensure that all airports, ports and roads remained open to humanitarian and civilian transit.

The United States is Georgia's principal military backer, and has provided significant quantities of military aid in recent years.