President Reagan yesterday reaffirmed the U.S. commitment to help Portugal modernize its military forces, while asking Portuguese Prime Minister Francisco Pinto Balsemao to help get a new U.S.-Portuguese agreement for American military facilities in the Azores.

After the morning meeting and a working lunch at the White House with Balsemao, the president read a statement saying that the two countries are at work on a new security arrangement to "broaden and strengthen our collaboration on our common defense objectives."

Reagan, addressing reporters outside the diplomatic entrance to the White House as Balsemao stood by his side, said the prime minister had informed him of Portugal's need for military modernization as well as economic assistance. The president said American aid to Portugal is "an important expression of our desire to befriend and help the Portuguese people."

Balsemao, 46, a former newspaper editor, read a statement in English after Reagan spoke. He said that although neogotiations between the two countries on the Azores agreement have just begun, the current political stability in Portugal increases the likelihood of continued cooperation with the United States.

"Portugal is a reliable partner," said Balsemao, "which wants to fully assume its responsibilities in secure terms, expects within this context a clear understanding from its American alliance."

The Reagan administration has asked $90 million in military aid for Portugal in the 1983 fiscal year, a $25 million increase in funding.

The prime minister, whose center-right Democratic Alliance coalition holds only a tenuous parliamentary majority against the opposition Socialists, said he had informed Reagan of the "recent political evolution" of Portugal, of the Portuguese economy and of its efforts to join the European Economic Community.