But in a boost for Ukraine, locked in a grinding struggle against Russia's invading army, the United States announced a $US700 million ($A976 million) weapons package for Kyiv that will include advanced rocket systems with a range of up to 80 kilometres.
Russia accused the United States of adding "fuel to the fire". Russian Foreign Minister Sergei Lavrov said the supply of the rocket launchers raised the risk of a "third country" being dragged into the conflict.
US Secretary of State Antony Blinken said Ukraine promised it would not use the systems to hit targets inside Russia. US President Joe Biden hopes extending Ukraine's artillery reach will help push Russia to negotiate an end to the war, now in its fourth month.
After days of heavy fighting around Sievierodonetsk, much of which has been laid to waste by Russian bombardments, Russian troops were inching forward through the city streets. Ukraine says about 70 per cent of the city is under Russian control.
"The enemy has entered the centre of Sievierodonetsk and is trying to take up positions," Ukrainian military spokesman Oleksandr Motuzyanyk said at a briefing.
If Russia captures the city and its smaller twin Lysychansk on the west bank of the Siverskyi Donets river, it would hold all of Luhansk, one of two provinces in the Donbas that Moscow claims on behalf of separatists.
Capturing all of Luhansk would fulfil one of Russian President Vladimir Putin's main aims and solidify a shift in battlefield momentum after his forces were pushed back from the capital Kyiv and from northern Ukraine.
Luhansk's regional governor, Serhiy Gaidai, told Reuters that civilians were sheltering from Russian attacks under a Sievierodonetsk chemical plant that he said was hit by an air strike on Tuesday, releasing a large pink cloud.
About 15,000 people remained in the city, Gaidai said.
Putin sent his troops over the border on what he calls a special military operation on February 24 to disarm and "denazify" Ukraine. Ukraine and its allies call this a baseless pretext for a war of aggression and the West has imposed stringent sanctions on Russia in a bid to strangle its economy.
Amid worries about the global ramifications of the war, Ukraine's grain traders' union said this year's wheat harvest was likely to drop to 19.2 million tonnes from a record 33 million in 2021.
Biden is due to meet NATO Secretary-General Jens Stoltenberg at the White House on Thursday. Stoltenberg told reporters he would convene a meeting in Brussels in coming days with officials from Sweden, Finland and Turkey to discuss Turkey's opposition to Sweden and Finland joining NATO.
Besides the advanced rocket systems, called HIMARS, the new US package includes ammunition, counter fire radars, air surveillance radars, additional Javelin anti-tank missiles and anti-armour weapons, officials said.
"The United States will stand with our Ukrainian partners and continue to provide Ukraine with weapons and equipment to defend itself," Biden said in a statement.
The decision to give Ukraine the rocket systems was made after Washington received assurances from Kyiv that it would not use them to hit targets inside Russian territory, which could broaden the war.
Ukraine has been seeking Multiple Rocket Launch Systems such as the M270 and M142 HIMARS to provide more firepower at longer range to hit Russian troop concentrations and weapons stockpiles at the Russian forces' rear.
The Pentagon said Washington would initially provide Ukraine with four HIMARS systems.
The new supplies come on top of billions of dollars worth of equipment such as drones and anti-aircraft missiles. The Biden administration plans to sell Ukraine four MQ-1C Gray Eagle drones that can be armed with Hellfire missiles for battlefield use against Russia, three sources told Reuters.
Kremlin spokesman Dmitry Peskov said the supplies would not encourage Ukraine's leadership to resume stalled peace talks.
