President Carter, in his first extensive decision-making on the U.S. role as arms merchant, has approved nearly $2 billion in military sales to NATO countries and other allies in Asia and the Middle East, administration sources said yesterday.

Close to 25 military sales were approved by Carter late last week out of a longer list of pending deals with a total price tag of $5 billion to $6 billion, according to officials. The items approved were described as relatively non-controversial, including howitzers and tanks for Israel, about $500 million in contract construction for Saudi Arabia, jet fighters for Greece and a variety of items for NATO countries, Jordan, Pakistan and South Korea.

Carter's decision will be announced soon and notifications of the proposed sales sent to Congress as required by law, officials said. The State Department is drawing up a justification for each transaction to be sent to Capitol Hill in keeping with a promise to lawmakers by Secretary of State Cyrus R. Vance.

Undersecretary of State for Security Assistance Lucy W. Benson, who took the oath of office at the State Department yesterday, said in an interview that Carter assumed a personal role in the review of each arms decision so far presented to him.

According to Benson, the Carter administration has adopted "very important" changes in its attitude and practices toward arms sales even presidential policy that is expected in mid April.

In the past the presumption was that the United States would sell a military item to allies and friends abroad in the absence of a showing that such a transfer would be against U.S. interest. Now policymakers have adopted a "negative presumption," according to Benson, placing the burden of justification on those who want a sale to go forward.

"The questions now are, 'Do they have to have this? Any why do they have to have this?'" Benson said. She said the change will produce a slow-down in arms sales almost automatically, even without the adoption of detailed criteria, procedures and anlytical tools envisioned in the proposed new presidential policy.

Benson said an interagency review of arms sale policy is scheduled for submission to the White House by April 11, and a National Security Council meeting on the question is expected two or three days thereafter. The announcement of a new presidential policy is likely to follow shortly after that, she said.

During the presidential campaign, Carter was sharply critical of existing U.S. policy in this field, charging that "almost completely unrestricted" sales of U.S. military weapons abroad was "a policy as cynical as it is dangerous."

Among the matters to be given consideration in the new arms policy will be the number of American technicians required for training and operation abroad of the arms that are proposed for sale, Benson said. She said consideration would also be given to the impact of arms sales on the domestic economy, including unemployment.