The Defence Secretary has accused Russia and Syria of carrying out attacks in Idlib "tantamount to war crimes" during a visit to the Pentagon.
Ben Wallace met with US Defense Secretary Mark Esper in Washington, to discuss pressing mutual security concerns and future relations between their two countries.
Topics including Syria, Russia, Afghanistan and China were all on the table during the meeting.
After an initial meeting, Mr Wallace insisted the regime of Syrian President Bashar Assad is intending to "flatten" the province.
"There’s an obligation on most of the civilised countries in the world to deal with what is a massively emerging humanitarian crisis in Idlib," said the British Defence Secretary.
"What is going on under the Assad and Russia regime is tantamount to war crimes."
This "humanitarian crisis" concerns Russian-backed attacks on Turkey - who support the rebels opposing the Syrian government and have themselves carried out acts of violence. It also concerns a migration crisis.
Turkey already has well over three million Syrian refugees, last week announcing it would not stop migrants from entering Europe. Tens of thousands have since gathered at the Greek border.
After President Assad's troops last month consolidated control of Aleppo, a continued offence toward the northeastern region aims to topple Idlib, the last remaining opposition stronghold.
"I think it is clearly the intention of the Assad regime to flatten Idlib and drive whoever’s left across the border and into Turkey," said Mr Wallace.
His comments came as Turkish President Recep Tayyip Erdoğan and Russian President Vladimir Putin reached an agreement on a ceasefire.
The deal will create a security corridor along the key M4 highway in northern Syria, where Turkish and Russian forces will launch joint patrols from 15 March.
Speaking alongside his American counterpart, Mr Wallace addressed recent concerns from former senior British military officials that a recent Afghan peace agreement could undermine historic troop sacrifice.
US and NATO forces based in Afghanistan agreed to withdraw from the country within the next 14 months after a peace deal was signed between the US and the Taliban.
"It's conditions-based, the numbers are going to be drawn down, they're not going to be all taken away, as the conditions change," Mr Wallace said.
"We still have a threat that we have to deal with - we will continue to deal with those threats.
"It is definitely a week-by-week, or a day-by-day examination of the Taliban adhering to those conditions, and indeed working with the Afghan government."
After being agreed, the conditional pact was soon tested, when a disputed prisoner swap deadline between the Afghan Security Forces and the insurgent group led to continued violence days later.
Having served with the Scots Guards throughout the 1990s, former Army officer Mr Wallace underlined that his regiment went to Afghanistan to stabilise the country.
"The biggest betrayal to the young men and women of all the allies who lost their lives - would be to not try [to] achieve peace," he said.
The deal brings to an end more than 18 years of conflict in Afghanistan, with 1,100 British troops still based there.
The US intervened in one of several post-deal attacks, launching a strike on Taliban forces and pushing the agreement into deeper waters.
Mr Esper maintained US commitment to allied Afghan forces and its right to defend them.
Although the UK has decided to include Chinese firm Huawei in its 5G networks, the US has taken the opposite path amid fears of cyber security and data sharing.
"Obviously, we disagreed with the approach," Mr Esper told UK reporters.
Allies adopting the Chinese technology will be assessed "on a case-by-case basis", he added.
Mr Wallace says a different "technical interpretation" to America's prompted the decision, rather than a differing view on China's threat level.
A "ban, cap and cut" direction by the UK was reaffirmed by the the UK's Defence Secretary, where Huawei is excluded from sensitive networks.
As Mr Wallace looked ahead to a Turkey visit next week, the pair stressed an enduring strategic relationship between the US and the UK.
During their meeting, the pair discussed the countries' parallel nuclear defence capability modernisation programmes.
Mr Esper expressed appreciation for the UK's leadership role in several areas, including at NATO and the country's ongoing contributions to international missions such as the International Maritime Security Construct.