A pregnant woman clutches her stomach as she is carried away from the ruins of a maternity hospital in southern Ukraine. It was hit by a Russian shell. 

Yesterday, her picture appeared in newspapers around the world. "Barbaric," declared one headline. Ukraine's president accused Russia of committing a war crime. 

Now, Western leaders are warning that worse may be to come. "We should all be on the lookout for Russia to possibly use chemical or biological weapons in Ukraine," said White House press secretary Jen Psaki. 

In recent days, Russian officials have claimed that the US is helping Ukraine develop chemical weapons. They could be trying to justify further attacks on Ukraine. 

Russia is already accused of multiple war crimes in Ukraine. Air strikes have hit schools, hospitals and homes.

The rules of war dictate what cannot be done in armed conflict. They aim to prevent needless brutality.

But proving war crimes is difficult. The ICC must prove that Russian forces deliberately targeted civilians, or knew that civilians would likely be harmed. 

Members of NATO have sent weapons to Ukraine, but the UK and US have ruled out enforcing a no-fly zone. Officials fear making the crisis worse, or even starting World War Three. 

Some think the use of chemical weapons, which are banned under international humanitarian law, could be a red line. NATO could change its mind and fight in Ukraine.

Yesterday, the UK armed forces minister said NATO "may intervene if Russia uses chemical weapons in Ukraine". 

"Putin seems hell-bent on escalation," added former British general Sir Chris Deverell. "Does NATO fight him now or fight him later?" 

Some say NATO is facing a terrible dilemma: stand back and let Ukrainians die or intervene and risk a world war. 

Yes: The world cannot stand by as young children are slaughtered. If bombing a maternity hospital is not a red line, then using chemical weapons should be. NATO must fight to end this tragedy. 

No: NATO is a defensive military alliance. Intervening in an attack on a third country risks making this conflict far worse and bringing conflict to peaceful nations. Fighting would be reckless. 

Or... There are more steps NATO can take before directly engaging in fighting. Member states should provide Ukraine with more weapons and increase sanctions on Russia - not send troops.