WASHINGTON (REUTERS) - The United States assesses that Russian strikes in western Ukraine in the past 24 hours were aimed at preventing airfields from being used by Ukrainian forces, a senior US defence official said on Friday (March 11).

The mayor of the city of Lutsk in western Ukraine, said four people were killed and six wounded in an attack on an airfield there, a rare strike on a target deep in western Ukraine far from the battlefields in the north, east and south.

The official, who spoke on condition of anonymity, could not say the extent to which targeted airfields had been used by Ukraine's air force.

Ukraine still had about 56 fighter jets available to use, more than 80 percent of their inventory, the official told reporters.

Russian jets were flying about 200 sorties per day in and around Ukraine, the official added.

Images released by private US satellite firm Maxar showed that a large Russian military convoy, last seen northwest of Kyiv near Antonov airport, has largely dispersed and redeployed.

The official said that some parts of the convoy had moved away from the main road and towards the tree line to protect themselves rather than a tactical move to advance towards Kyiv.

Advance elements of the Russian military were as close as about 15km away from Kyiv's city centre, the official added.