LISBON, Feb 23 (Reuters) - Portugal's Foreign Minister Paulo Rangel said on Monday that the sudden increase in U.S. aircraft activity at the Azores' Lajes air base is governed by a decades-old bilateral treaty and does not require Lisbon's permission.

Portugal’s left-wing opposition has called on the center-right government to clarify the legal basis behind the increase in US military flights and to explain whether Lisbon had given its approval.

Washington has embarked on what it describes as its biggest military buildup in the Gulf since the 2003 Iraq war, as hopes fade for a diplomatic solution to Iran's nuclear program.

Speaking to reporters in Brussels, Rangel said that US aircraft had "increased their use of the Lajes base in the last few weeks," but without any breach of the rules agreed upon between Portugal and the United States under the 1951 treaty.

He added, "(Increased use) does not require permission from Portugal nor notification of the flag."

The minister added, "Look at the past fifty or sixty years and you will see that it has always been this way... and it will continue to be this way. We are fulfilling our commitments."

Rangel said that Portugal has consistently called for diplomatic solutions to international tensions, including those related to Iran, and advocates for the peaceful settlement of conflicts.

The Lajes base, located in the Azores archipelago in the Atlantic Ocean, has always been a strategic transit point for US forces.

He said, "Portugal has always defended its alliance with the United States and its membership in NATO, and that has not changed with the geopolitical shift."