KABUL, Afghanistan — A German engineer held hostage by the Taliban for more than a month appeared on Afghan television yesterday, coughing and holding his chest while appealing for help.

Taliban militants, meanwhile, ambushed a convoy of supplies for NATO-led troops in southern Afghanistan, killing 10 Afghan guards, an official with the convoy said.

The trucks were attacked near Qalat city, in Zabul province, as they were traveling on the main Kabul-Kandahar highway, said Mohammad Salim, an official with the private security company, who witnessed the attack.

The hostage shown on privately owned Tolo TV was one of two German engineers and five Afghans abducted July 18 in Wardak province in central Afghanistan. The other German was found dead of gunshot wounds July 21, while one of the Afghans managed to escape.

“I am a prisoner of the Taliban,” said the man, who identified himself as Rudolf Blechschmidt. “We live in the mountains, very high in a very bad condition, please help us.”

Tolo TV did not say how it obtained the video, and there was no indication of when it was shot.

The German Foreign Ministry said it was checking the video’s contents.

“The Foreign Ministry’s crisis task force is carefully analyzing the video,” a spokeswoman for the Foreign Ministry said on customary condition of anonymity. “It continues to work intensively to secure the hostage’s release.”

One of the Afghan hostages also appeared on the video, asking for the government’s help.

“Think of our children,” the unidentified man said. “Only God and [President] Karzai can help us now.”

Mr. Blechschmidt was filmed on his side on a black rug. He appeared to be in pain, stopping to cough deeply and clutching his chest.

“The Taliban try to negotiate with the Afghan government but the government not talk with the Taliban and the Taliban tried to get in connection with the embassy to release us. But if the time is over, they want kill us,” he said, speaking in broken English.

The captors have demanded in the past that Germany withdraw its troops.

Abductions have become a key insurgent tactic in recent months in trying to destabilize the country, targeting both Afghan officials and foreigners helping with reconstruction. A group of 23 South Korean aid workers were taken hostage last month. Two were killed, two were released, and the rest remain captive.

In violence-plagued Helmand province, a bomb next to a convoy carrying Police Chief Mohammad Hussein killed three passersby and wounded 13 others, he said.

The bomb was triggered by remote control in the town of Gereshk, said Mr. Hussein, who was unhurt. Five of the wounded were in critical condition, he said.