In a separate clash, Afghan and coalition soldiers killed seven militants in the southern province of Uruzgan, where a large-scale anti-Taliban offensive is under way, an Afghan official said.
Officials said the first convoy attack, initially reported on Sunday, was on vehicles carrying a former district chief in Helmand province, leaving the ex-official and four of his bodyguards dead.
The second ambush occurred several hours later, when about 40 of the slain official's relatives went to collect his body, his brother Dad Mohammed Khan said.
Khan, a parliamentarian and former Helmand intelligence chief, said 25 people were killed, including the brother and nephew of the dead official, Jama Gul. Four others were wounded and 10 others were missing.
"We don't know whether they are dead or alive," Khan said.
Afghan army commander General Rahmatullah Roufi said that on Sunday, US-led troops, backed by Afghan forces, raided a Taliban mountain stronghold near Uruzgan's provincial capital and killed seven militants.
After three hours of fighting, four wounded militants were arrested while others escaped into the mountains, Roufi said. Troops confiscated 11 AK-47 rifles, six rocket-propelled grenades and four machine guns.
Uruzgan is one of four southern provinces where more than 10,000 US-led troops are hunting Taliban militants in an effort to quell a militant resurgence. Operation Mountain Thrust is the largest military offensive since the Taliban government was overthrown in 2001.
More than 600 people, mostly militants, have been killed in the past month as insurgents launched increasingly bold attacks against coalition forces.
Since last week, about 100 suspected militants have been killed in the offensive. At least nine coalition soldiers have died in combat since mid-May.
On Sunday, US soldiers moved deep into remote southern Afghan mountains, setting up positions aimed at blocking Taliban movement and supply routes.
The United Nations on Monday urged Afghan and US-led forces to avoid harm to civilians during the offensive.
"While these operations are ongoing, it's imperative that the security forces - both national and international - exercise extreme caution to prevent any civilian casualties," UN spokesman Aleem Siddique told reporters.
"We call on the security forces to do everything they can to ensure the safety and protection of the local communities in districts where the operations are taking place," he said.
Up to 34 civilians, including women and children, were killed in mid-May when US warplanes seeking militants bombed a village outside Kandahar.