Officials from the two Koreas met Monday in North Korea to discuss their joint industrial complex just days after an exchange of gunfire at sea emphasized the constant security threat on the divided peninsula.

North Korea lobbed dozens of shells toward the western sea border last week, prompting South Korea to respond with a barrage of warning shots. Pyongyang called it a military exercise, and South Korean officials reported no casualties or damage.

Two no-sail zones ordered by North Korea early last week just before the fracas remain in place, and on Monday the Yonhap news agency in Seoul said Pyongyang issued notices for five new no-sail zones: four off the west coast and one off the east.

The poorly marked sea border is a constant source of tension between the Koreas. Their navies fought a skirmish in November that left one North Korean sailor dead and three others wounded, and engaged in bloodier battles in the area in 1999 and 2002.

Despite the flare-up in tensions, officials met at the North Korean border town of Kaesong as scheduled to discuss how to further develop their joint factory park.

Talks ended Monday without any agreement, according to Seoul's Unification Ministry. The North repeated a demand to put wage hikes on the agenda, while the South said discussions must focus on easing border crossings for South Korean workers, ministry spokeswoman Lee Jong-joo said.

The sides also failed to set a date for the next round of working-level talks on the Kaesong complex, she added.

Since 2004, Kaesong has combined South Korean capital and know-how with cheap North Korean labor — a key symbol of cooperation between the wartime rivals.

Tensions last year between the Koreas, which technically remain in a state of war because the two have not signed a peace treaty, put the project in jeopardy.

On Monday, the North repeated its call for a peace treaty, criticizing Washington for raising tensions by keeping troops in South Korea and cementing its military alliance with Seoul.

The U.S. policy is "nothing but an attempt to stifle (North Korea) by force and to hold unchallenged military hegemony in the region," North Korea's main Rodong Sinmun newspaper said in commentary carried by the official Korean Central News Agency.

North Korea maintains it was forced to develop its nuclear weapons program as a defense against the U.S. military presence in rival South Korea. Last year, it quit ongoing six-party negotiations aimed at persuading it to give up its nuclear arsenal in exchange for aid.