Ukraine has fired US-made long-range missiles deep into Russia for the first time in a major escalation of the war, as Vladimir Putin issued his own nuclear warning.

The US ATACMS missiles struck Russian territory on day 1000 of the conflict, two days after US President Joe Biden approved their use.

Russia said its forces shot down five of six missiles fired at an ammunition in the western Bryansk region while debris from one hit the depot, causing no casualties or damage.

Ukraine said it had struck a Russian arms depot about 110 kilometres inside Russia and caused secondary explosions.

It did not specify what weapons it had used.

Russia described the attacks as an escalation that would make the US a direct combatant in the war and prompt retaliation.

Putin on Tuesday (local time) changed his country’s nuclear doctrine by lowering the threshold that would prompt Russia to consider using nuclear weapons.

The new policy says any conventional assault on Russia by a non-nuclear power supported by a nuclear power will be considered to be a joint attack.

Any mass aerospace attack with aircraft, cruise missiles and unmanned aircraft that crossed Russia’s borders could also trigger a nuclear response, it says.

It came amid plans for vigils to mark 1000 days of war, with weary troops at the front, Kyiv besieged by air strikes and doubts about the future of foreign support as Donald Trump heads back to the White House.

Military experts say US missiles can help Ukraine retain a pocket it has captured inside Russia – in the Kurk region – as a bargaining chip but are not likely to change the course of the 33-month-old war.

Potentially more consequential changes in the US posture are expected when Trump returns to power in two months, having pledged to end the war quickly without saying how.

In an address to parliament, Ukrainian President Volodymyr Zelensky said the war’s “decisive moments” would come in the next year.

“At this stage of the war, it is being decided who will prevail. Whether us over the enemy, or the enemy over us Ukrainians … and Europeans. And everyone in the world who wants to live freely and not be subject to a dictator.”

Zelensky set out what he called a “resilience plan” for the country as a domestic foil to the “victory plan” he pitched to allies earlier. He said it was needed to force Russia to negotiate an end to the war in good faith.

Although he said the full details of the plan would be disclosed later in the year, it comprises steps to stabilise the front line, support military innovation and arms production as well as measures to shore up unity and cultural identity.

“Unity is the first point of our internal resilience plan,” he said.

Zelensky said Ukraine would produce at least 30,000 long-range drones next year, a weapons system that the country’s forces have used to narrow the gap in strike capabilities with Russia and to strike targets deep inside its much larger eastern neighbour.

Putin lowers nuclear threshold

Putin signed a revised nuclear doctrine declaring that a conventional attack on Russia by any nation that was supported by a nuclear power would be considered a joint attack on his country.

The signing of the doctrine, which says that any massive aerial attack on Russia could trigger a nuclear response, demonstrates Putin’s readiness to tap the country’s nuclear arsenal to force the West to back down as Moscow presses a slow-moving offensive in Ukraine.

Asked whether the updated doctrine was deliberately issued on the heels of Biden’s decision, Kremlin spokesperson Dmitry Peskov said on Tuesday the document was published “in a timely manner” and Putin instructed the government to update it earlier in 2024 so it was “in line with the current situation”.

Putin announced changes in the nuclear doctrine in September, when he chaired a meeting discussing the proposed revisions.

Russia’s president has previously warned the US and other NATO allies that allowing Ukraine to use Western-supplied longer-range weapons to hit Russian territory would mean that Russia and NATO were at war.

The updated doctrine states that an attack against Russia by a non-nuclear power with the “participation or support of a nuclear power” will be seen as their “joint attack on the Russian Federation”.

It adds that Russia could use nuclear weapons in response to a nuclear strike or a conventional attack posing a “critical threat to sovereignty and territorial integrity” of Russia and its ally Belarus, a vague formulation that leaves broad room for interpretation.

It does not specify whether such an attack would necessarily trigger a nuclear response.

-with AAP