SEOUL (Reuters) - South Korea s President Moon Jae-in and Japanese Prime Minister Shinzo Abe agreed on Monday to pursue stronger United Nations sanctions against North Korea after Pyongyang s sixth nuclear test a day earlier, South Korea s presidential spokesman said.  Both heads of state agreed to cooperate closely with each other and the United States and shared the understanding there must be the most powerful sanctions and pressure applied on North Korea,  presidential Blue House spokesman Park Su-hyun told a media briefing.  And as part of that they agreed to push for more powerful U.N. sanctions,  Park said after Moon and Abe spoke for about 20 minutes by phone. The aim of stronger sanctions was to draw North Korea into dialogue, he said. 