Ukraine's army accused Russia of carrying out strikes using incendiary phosphorus munitions on Snake Island Friday, just a day after Moscow withdrew its forces from the rocky outcrop in the Black Sea.

"Today at around 18:00... Russian air force SU-30 planes twice conducted strikes with phosphorus bombs on Zmiinyi island," it said in a statement, using another name for Snake Island.

The Russian defence ministry on Thursday described the retreat as "a gesture of goodwill" meant to demonstrate that Moscow will not interfere with UN efforts to organise protected grain exports from Ukraine.

The Ukrainian army on Friday accused the Russians of being unable to "respect even their own declarations".

Its statement was accompanied by a video that showed a plane drop munitions at least twice on the island, and what appeared to be white streaks rising above it.

Phosphorus weapons, which leave a signature white trail in the sky, are incendiary weapons whose use against civilians is banned under an international convention but allowed for military targets.

Ukraine has accused Russia of using them several times since it invaded its neighbour in late February, including on civilian areas, allegations Moscow has denied.

Ukraine claimed the Russians were forced to retreat from the island after coming under a barrage of artillery and missile fire.

Snake Island became famous after a radio exchange went viral at the start of the war, in which Ukrainian soldiers respond "go f*ck yourself" to a Russian warship that called on them to surrender.

US announces more missiles, ammunition

The Pentagon announced Friday $820 million in additional weapons and ammunition for Ukraine as it battles Russian forces along the eastern and southern fronts.

The 14th package of armaments for Ukraine forces includes two air defense systems, more ammunition for the Himars precision rocket launchers the US began supplying in June, up to 150,000 rounds of 155mm artillery ammunition, and four additional counter-artillery radars.

The air defense systems, known as NASAMS, are a short and medium-range surface-to-air missile system developed by Raytheon and Norway's Kongsberg Defence & Aerospace. 

The remote-operated launchers are to help Ukraine forces defend against piloted and drone aircraft and cruise missiles.

"The United States continues to work with its allies and partners to provide Ukraine with capabilities to meet its evolving battlefield requirements," said Defense Department Press Secretary Todd Breasseale in a statement.

The Pentagon "recognizes Norway's cooperation to enable the historic provision by the United States of modern air defense systems that will help Ukraine defend against Russia's brutal air attacks," he added.

The new package of arms, which draw on the US military's own stockpiles, took to $6.9 billion the total amount provided by Washington to Ukraine since Russia invaded the country on February 24.