Gunmen on a motorbike shot dead two Russian military instructors in the Yemeni capital on Tuesday, a Yemeni police source said.
The source said the instructors, who worked with the Yemeni army, were shot as they left a hotel where they were staying in the southern part of the capital.
Dubai-based al-Arabiya television reported that one Russian military adviser was killed and another was wounded in the attack.
An official at the Russian embassy said he could not confirm that Russians had been killed in the incident but said there were no Russian military instructors currently in Yemen.
The Russian Defence Ministry could not immediately be reached for comment.
The Yemeni army had long relied on foreign advisers as it sought to quell militant elements in parts of the country but it has phased out most of that program and only a handful remain.
It was not immediately clear who was behind the attack. There have been repeated drive-by shootings using motorbikes since an uprising that forced long-serving President Ali Abdullah Saleh to step down in 2011. Authorities have blamed these attacks on Islamist militants linked to al Qaeda.
In October last year, two masked men on a motorcycle shot dead Brigadier General Khaled al-Hashemi, an Iraqi army officer who worked as a consultant at the Ministry of Defence.
The United States is worried that al Qaeda, entrenched in parts of Yemen, will use a power vacuum to launch attacks abroad, and has stepped up drone strikes on suspected militants with the backing of President Abd-Rabbu Mansour Hadi.