Scores of veterans of elite Israeli intelligence unit 8200 rallied to its defence Sunday after 43 comrades said they would no longer take part in its "injustices" against millions of Palestinians.

The open letter, which was sent to Israel's political and military leadership last week, was one of the most high-profile expressions of conscientious objection in years.

The signatories -- reservists and former members of 8200 -- said the intelligence collected by the unit "was an integral part of Israel's military occupation", and that they would refuse to continue to serve.

They charged that information gathered by Unit 8200 was used by civilian intelligence agencies to coerce Palestinians uninvolved in militant activity, and urged other members of the intelligence corps "to speak out against these injustices and to take action to bring them to an end".

But in a letter of reply on Sunday, 200 veterans of the unit denounced their former comrades' refusal to serve.

"We wish to express shock, disgust and complete disassociation from the regrettable letter that was written by our comrades from the unit," they wrote in the letter, excerpts of which were published in the top-selling Yediot Aharonot newspaper.

"Political refusal to serve has no place anywhere, and particularly so in Unit 8200. The moment we, as soldiers in the reserves, are called to the flag, we set aside our political inclinations and opinions, and come to serve the state."

Prime Minister Benjamin Netanyahu said any form of refusing military service was "condemnable", calling the refuseniks' move "political" and based on "unfounded accusations".

"This is unacceptable," he said at a cyber security conference in Tel Aviv.

Commentators said that the fact that the refuseniks were members of one of Israel's most prestigious military units made their conscientious objection all the more remarkable.

"The Unit 8200 letter represents a watershed moment in the expressions of military refusal in recent decades," wrote Shimon Shiffer in Yediot Aharonot.

"This time, we are talking about intelligence gatherers who are refusing to spy on millions of Palestinians."

Unit 8200 carries out electronic communications monitoring and surveillance, similar to work performed by the US National Security Agency and Britain's GCHQ.

The unit is one component of the broader military intelligence corps and shares information with Israel's civilian intelligence agencies.

The refuseniks' letter drew criticism from both the government and the opposition.

Defence Minister Moshe Yaalon described it as a "foolish and offensive attempt" to harm the unit.

Opposition leader Isaac Herzog, himself a veteran of Unit 8200, said he opposed soldiers refusing to serve.

"I'm not saying that there are no mistakes. It is certainly possible that there were," he wrote on his official Facebook page.

"But there are ways to complain and ensure such claims are examined and discussed," he said.