The Biden administration delayed a request from Ukraine to provide longer-range missiles over fears it could provoke a lethal response from Russia - senior Pentagon officials have opposed the idea, according to two military officials speaking to NBC News.

The missiles in question are the ATACMs, or the Army Tactical Missile Systems, which military officials have refused to supply Ukraine with. The officials were concerned that the missiles would be used against targets within Russian territory and would widen the scale of the war with Russia.

Russian Foreign Ministry spokeswoman Maria Zakharova warned Washington against providing the missiles to Ukraine, dubbing it a "red line."

“If Washington decides to supply longer-range missiles to Kiev, then it will be crossing a red line, and will become a direct party to the conflict,” Zakharova said. 

On Thursday, the Biden administration announced that Washington will be sending another $600 million in military assistance to Ukraine on top of the $675 million package which was announced last week.

The Pentagon, later on, divulged that the package will include more "ammunition for High Mobility Artillery Rocket Systems (HIMARS)," thousands of 105mm and precision-guided 155mm artillery rounds, counter-artillery radars and anti-drone systems. Anti-personnel mines, small arms and ammunition and night vision goggles, and cold weather gear will also be included.

Although a number of lawmakers from the Republicans and Democrats support Ukraine's request for longer-range missiles, which could travel up to 300 km, Biden said last month that Ukraine does not need the ATACMs, arguing that the missiles at the present time have proven effective against Russian forces. 

On Tuesday, Biden reiterated that his administration is "not going to send to Ukraine rocket systems that strike into Russia.

“It’s not on the table at this time,” a senior US official said Friday, stating that dynamics on the battlefield can change “as their needs evolve, the types of assistance evolves.”

On Thursday, the Pentagon announced a $311 million contract to Lockheed Martin and Raytheon to replenish stocks of depleted Western-supplied Javelin anti-tank missiles.

The Biden adminsitration, so far, has provided Ukraine with $44.3 billion in aid - while the aid is only partially in the form of cash, the rest is weapons, ammunition and training.