Ukraine's President Zelensky urged Europe to "push Russia harder" in a speech marking 1,000 days since Vladimir Putin launched a full-scale invasion of the country.

"The more time he [Putin] has, the worse the conditions become," he told a special session of the European Parliament on Tuesday.

Zelensky spoke amid speculation that Ukraine had for the first time used the US-supplied Atacams long range missile system to hit a target deep inside Russia, a day after Washington signalled its permission.

An ammunition warehouse was struck with the missiles in the Russian region of Bryansk, about 100 kilometres from the border, an unconfirmed report on the RBC Ukraine news website said.

But Kyiv often uses domestic drones to hit targets inside Russia, and there was no immediate evidence that US missiles were used.

On Monday, US officials said that President Biden had removed US sanctions on the use of the Atacms system outside of Ukraine's borders.

It is thought that Ukraine was given permission to use the missiles only to defend its forces inside Russia’s Kursk region, where Kyiv launched a surprise incursion in August and where an assault from Russian and North Korean troops was expected within days.

Addressing members of the European Parliament on Tuesday morning, 1,000 days since the invasion began on 24 February 2022, Zelensky warned that Putin "[would] not stop on his own" and urged that Russia be pushed towards a "just peace".

He said the deployment of North Korean troops was a clear sign that Moscow was determined to escalate the conflict.

"While some European leaders think about some elections, or something like this, at Ukraine's expense, Putin is focused on winning this war," he warned.

Meanwhile, Ukraine's military commander in chief wrote on Telegram that the country had faced 1,000 days of "extremely complex, fierce battle for our existence" and "destroying the enemy".

"In the frozen trenches of Donetsk region and in the burning steppes of Kherson region under shells, hail, and anti-aircraft guns - we are fighting for the right to life."

He added: "Every dark night, even if there are a thousand of them, always ends with dawn."

The comments came after 12 people, including a child, were killed in a drone attack on Ukraine's northeastern region of Sumy overnight, where 11 were also killed in a separate strike on Sunday.

Meanwhile, the Kremlin approved its updated nuclear doctrine - which says that any aggression from a non-nuclear state, if backed by a nuclear power, will be regarded as a joint attack on Russia.

Under the changes, a large attack on Russia with conventional missiles, drones or aircraft could meet the criteria for a nuclear response, as could an attack on Belarus or any critical threat to Russia's sovereignty.

The EU's foreign affairs chief Josep Borrell Fontelles also said on Tuesday the EU continued to stand with Ukraine, but that it "need[ed] to do more and quicker".

"The European Union will continue to advance support to help achieve victory for Ukraine and to bring peace to our continent," he said in a video shared on X.

"History will judge us based on our actions and reactions," read the caption.

Earlier, UK Prime Minister Sir Keir Starmer reiterated his country's "ironclad" support for Ukraine amid speculation he could give Kyiv permission to use British-supplied Storm Shadow missiles to hit targets deep inside Russian territory.