AMNA NAWAZ: Since last fall, more than 12,000 North Korean servicemen have reportedly been deployed to fight alongside the Russian army in Ukraine.

More recently, Ukraine has accused the Kremlin of recruiting foreign fighters from an entirely different region.

Growing numbers of mercenaries from Cameroon, Ghana, Senegal, Uganda and other African nations reveal the extent to which Russia is struggling to recruit fresh troops among its own population.

Reporting from Ukraine, special correspondent Simon Ostrovsky investigates how these mercenaries also represent an increased reliance on guns for hire in some of the world's deadliest conflicts.

SIMON OSTROVSKY: This is one of five prisoner of war camps in Ukraine, and the Ukrainian authorities won't tell us how many POWs they have.

But what we do know is that there's a mix of Russians who were captured, as well as foreign nationals from third countries.

And, increasingly, many of those fighters have been from Africa.

A vast Kremlin-linked network of recruiters is pulling in thousands of mercenaries from across the African continent to fight and die for Russia and Ukraine.

The lucky few who survive end up here, where one in 10 are foreign fighters according to Ukrainian officials.

But the ramifications of what's happening go far beyond this European theater of war.

The African recruits and other internationals are helping Russia increase its battlefield numbers.

But that's not just a problem for Ukraine.

They're leaving their homelands in such high numbers, particularly in some countries in Africa, that it's becoming a security issue there.

So many troops from Cameroon have joined up that the Cameroonian Defense Ministry ordered its officers to -- quote -- "immediately take appropriate measures against desertion."

All men in uniform are now banned from traveling abroad without special permission, according to this March document, which was provided to "News Hour" by Atlantic Council fellow Alia Brahimi.

ALIA BRAHIMI, Atlantic Council: The Cameroonian government are feeling vulnerable.

They're fighting on four fronts at least, pirates on the coast, Central African rebels in the east, English-speaking separatists in the south, ISIS and Boko Haram that are on the march in the north of the country.

They can't afford to be losing any soldiers, let alone their most skilled soldiers, let alone their special forces, who we know are going to Ukraine, they're fighting in Ukraine, and they're dying in Ukraine.

MAN (through translator): All good.

All good.

I'm from Cameroon.

I came to Russia.

Russia.

SIMON OSTROVSKY: While it's hard to know exactly how many men have enlisted, reportedly, as many as 100 are dead or missing in action.

Their loved ones now search for them across social media, posting pictures like these, in the hopes that they will be identified.

Petro Yatsenko from Ukraine's coordination headquarters for the treatment of prisoners of war says that, over the past few months, he's seen an increase of foreign fighters at camps like this one.

We know why the Russian government is recruiting foreigners to fight in its military.

They need more men.

But why are these people agreeing to fight for Russia?

I mean, the numbers of people being killed on the battlefield is huge.

PETRO YATSENKO, Coordination Headquarters for Treatment of Prisoners of War: Russia needs more and more cannon fodder.

They never say the truth for these foreign mercenaries.

They promise them very big salaries.

They promised them to be very far from the front line.

In fact, they were thrown to the front line, to the kill zone, to the very hard conditions to survive.

SIMON OSTROVSKY: Mohammed Imraaj (ph) 26, is originally from Egypt.

He says an army recruiter promised him a Russian passport.

But after just three days of fighting, he ran out of ammo and was captured, along with another mercenary from Egypt, but not before seeing his comrade die right next to him.

MOHAMMED IMRAAJ, Former Mercenary: It was first time to me.

It was too much hard, no.

And you see him, like, just die and you can't do anything.

You can't help him.

You can't do anything.

SIMON OSTROVSKY: Ukrainian military officials report a flood of mercenaries like these two men on the front lines in the past year and say most don't survive more than a month.

Recruited from countries such as Uganda, Kenya, Togo and South Africa, over 1,400 citizens of 36 African states are currently fighting for Russia, according to Ukraine's Foreign Ministry.

Russian recruiters target active-duty servicemen across African countries, promising salaries 10 times what they make at home, sign-up bonuses upwards of $2,000, and even Russian passports.

WOMAN (through translator): A Russian man from Cameroon will go serve in the special military operation.

SIMON OSTROVSKY: And it's not just Russia that's growing more dependent on mercenaries in private military companies, or PMCs.

Brahimi warns of a growing mercenary storm, with Russia's tactics capturing the attention of both American defense firms and the White House.

Reportedly, the Trump administration is considering private sector solutions for problems as varied as peacekeeping in Ukraine to security along America's southern border.

In Gaza, the White House has already come under fire for using military contractors to distribute food aid.

ALIA BRAHIMI: It's part of this broader phenomenon where the private security industry is now a go-to option to solve thorny security problems.

SIMON OSTROVSKY: So are we seeing the Trump administration echo what Russia has used PMCs for?

ALIA BRAHIMI: This very much fits in with the general ethos of the Trump administration and particularly his commitment to ending U.S.

military commitments abroad.

SIMON OSTROVSKY: And, in Ukraine, the number of foreign combatants keeps climbing.

The vast majority of captured foreign fighters end up in legal limbo, with neither Russia nor their home countries interested in trading for them.

That leaves these men stuck in prisons for months, even years.

Richard Kanu is a 42-year-old former soldier from Sierra Leone who was captured after two weeks of fighting.

That was more than a year-and-a-half ago.

So what would you say to other young men from Africa who are thinking about coming to Russia to join the military?

RICHARD KANU, Former Mercenary: Even when I'm here now, still, Africans are signing contact for Russians, because, for now, we have no hope.

We don't have someone who can plead on our behalf how to get out of this place,.

And this is not a life I want to live.

SIMON OSTROVSKY: But as long as there are young men from some of the world's poorest countries dreaming of escaping the life at home, there will be no shortage of men fighting someone else's war for pay.

For the "PBS News Hour," I'm Simon Ostrovsky in Western Ukraine.