China on Thursday threatened to make a “legitimate and necessary” retaliation over the US sale of $1.8 billion worth of arms to Taiwan as Beijing becomes more strident over its claims to the self-ruled island.

The US state department said on Wednesday it has approved the sale of 135 air-to-ground missiles to Taiwan in a move Taipei’s defence ministry said would build its combat capabilities.

“We will not engage in an arms race with the Chinese Communists. We will put forward requirements and build fully in accordance with the strategic concept of heavy deterrence, defending our position and defensive needs,” defence minister Yen De-fa said.

Self-ruled Taiwan lives under constant threat of invasion by China, whose leaders view the island as part of their territory.

China’s foreign ministry on Thursday accused the US of violating agreements signed by Beijing and Washington in the 1970s establishing diplomatic relations between the two governments.

The sale is “sending a very wrong signal to separatist forces advocating for Taiwan independence, and seriously damages China-US relations,” ministry spokesman Zhao Lijian said.

China also promised retaliation after US secretary of state Mike Pompeo said the previous day the state department was designating the US operations of six more China-based media companies as foreign missions.