For stories on Russian bounty payments for American troops in Afghanistan--especially the credibility of the reports, the legality of such actions, and the multi-country history of bounties in foreign policy--contact Mary Ellen O'Connell, Robert and Marion Short Professor of Law and research professor of international dispute resolution at University of Notre Dame. 

https://law.nd.edu/directory/mary-ellen-oconnell/

O'Connell is an expert in international human rights law and armed conflict, and she says: 

President Trump and his national security team are denying reports in the New York Times, Washington Post, and elsewhere of Russian payments to the Taliban and their associates to kill American troops in Afghanistan.  

The reports appear credible and the practice is consistent with Russia current foreign policy. The President needs to condemn the practice and needs no absolute proof to do so.  

Paying non-state actor armed groups to kill government troops violates international law. 

But then the U.S. has engaged in similar conduct for decades, undermining the rules against it.

In 1979, the Soviet Union invaded Afghanistan and fought the Mujahideen for a decade. The U.S. funneled resources to the opponents of the Soviets, some of whom went on to create Al Qaeda, including Osama bin Laden. 

The invasion of Afghanistan violated international law but aid to non-state actors armed groups was not the lawful way to oppose such a breach. The unintended negative consequences could have been predicted. Nevertheless, the U.S. has continued with the unlawful practice, taking it to new lows in aiding non-state actor armed groups against government troops in Syria and Libya.  

The payment of bounties to kill our troops is an egregious law violation--the President can say so, no matter what he thinks of the intelligence or the news media that report on it.