North Korea and the United States have no diplomatic relations and virtually no official channels of communication.
Tokyo: A North Korean parliamentary committee sent a rare letter of protest to the US House of Representatives on Friday over its new package of tougher sanctions.
The sanctions were condemned as a "heinous act against humanity" by the foreign affairs committee of the North's Supreme People's Assembly, according to a state media report.
It was not immediately clear how the protest was conveyed - if it was sent by mail or how it was addressed - since North Korea and the United States have no diplomatic relations and virtually no official channels of communication. The report, carried by the North's Korean Central News Agency, said the letter was sent Friday.
The Republican-led House overwhelmingly voted May 4 to impose the new sanctions, which target North Korea's shipping industry and use of what the bill called "slave labour."
It's not unusual for Pyongyang to condemn Washington's moves to censure it, but direct protests to Congress are exceptionally rare. Pyongyang normally expresses its displeasure with Washington through statements by the Foreign Ministry or other institutions, or through representatives at its United Nations' mission in New York.
Koh Yu-hwan, a North Korea expert at Seoul's Dongguk University, said it's not unprecedented for the North to directly contact the US legislature or government. Pyongyang sent letters to the United States in 1984 calling for the opening of three-way talks between Pyongyang, Seoul and Washington.
But he said Friday's protest was also notable in that it was sent by the recently revived parliamentary foreign affairs committee, which was discontinued by Kim Jong Un's father, Kim Jong Il, in 1998.
The move to restart the committee has been seen as an attempt to create a "window" for contacts with the outside world - Seoul and Washington in particular.
Pyongyang has had more than its usual share of criticism for Washington in recent months.
It has been in a high-indignation mode for the past two months because of military exercises between the US and South Korea that it sees as a prelude to invasion. This year's war games were the biggest ever, and reportedly included training for precision strikes and assaults intended to take out Kim Jong Un and his top lieutenants.
The North also announced last week that it thwarted what it claims was a CIA-backed attempt to assassinate Kim. On Friday, its Central Public Prosecutor's Office issued a statement suggesting the United States and South Korea are harbouring suspects and should extradite them to the North immediately.
At the same time, however, a senior North Korean Foreign Ministry official reportedly flew to Oslo, Norway, to meet with former US diplomats and scholars in what is known as "track 2" talks on bilateral issues. The talks, which are held intermittently, are an informal opportunity for the two sides to exchange opinions and concerns.
The US Senate would need to approve the new sanctions next before they could be implemented. The House bill bars ships owned by North Korea or by countries that refuse to comply with UN resolutions against it from operating in American waters or docking at US ports.
Goods produced by North Korea's forced labour would be prohibited from entering the United States, according to the legislation.