SEOUL, July 7 (Xinhua) -- North Korea has increased the number of threats it has issued against the United States, but it has also sent 95 emails and faxed messages to the South requesting a joint conference on unification in August.

Pyongyang's state-controlled news agency KCNA said Tuesday the United States and Japan launched a "criminal joint nuclear attack training exercise" in May, citing the presence of B-52 Stratofortress bombers near South Korea in June.

The United States is set on "maximizing" the risk of nuclear war on the Korean peninsula and therefore is in no position to advocate denuclearization, Pyongyang said.

An article published Saturday in the Workers' Party Rodong Sinmun read, "Even if our sovereignty and right to exist are interfered with at the slightest, we will turn the U.S. imperialist military bases into seas of fire."

North Korea propaganda outlet DPRK Today has also uploaded videos of medium-range ballistic missile launches, and simulations of attacks against the U.S. mainland or U.S. military bases, Yonhap reported.

Ahn Chan-il, a North Korean defector and head of the World Institute for North Korea Studies, said the rising verbal attacks are a sign the Kim Jong Un era has officially begun, and to mark the period of his rule, Pyongyang is "showing off" its foreign and South Korea policy.

Threats, however, have also been accompanied by overtures to Seoul, calling for talks on unification to commemorate the 71st anniversary of independence from Japanese colonial rule.

According to the unification ministry, the government has received 95 messages calling for talks, but the North must first show a willingness to stop provocations and move toward denuclearization, Seoul said.