Islamist militants attacked police posts in Pakistan's northwest and killed two civilians active in an anti-Taliban militia, challenging a security establishment straining under a national flooding disaster, police said Wednesday.The attacks came as the UN said an estimated 4.6 million flood victims have yet to get any shelter, despite aid workers' attempts to distribute tents. The floods have submerged tens of thousands of villages, killed around 1,500 people and affected 20 million others, authorities say.A group of militants first killed two members of a militia in the Adezai area of Peshawar as they headed to pray at a mosque late Tuesday, said Liaqat Ali Khan, Peshawar police chief.In the hours after, dozens of militants from the Khyber tribal region, which lies near Peshawar and along the Afghan border, attacked police posts in the Sarband area of Peshawar. The two sides exchanged fire for about an hour before the militants retreated to Khyber, Khan said.He said several militants were killed, but there were no police casualties.The clashes suggest Islamist insurgents are not abandoning their campaign against the state despite the flooding that began three weeks ago. In fact, they may be taking advantage of the government's weak and distracted status."As the police force is busy in rescue and relief work for flood affectees, militants tried to take advantage of the situation to attack Peshawar, but the police force was fully alert and vigilant," Khan said. Pakistani President Asif Ali Zardari was headed to Russia on Wednesday for a regional summit. He was expected to stay only a few hours before returning to his deluged country. An earlier multi-day trip to Europe just as the disaster was unfolding severely damaged Zardari's already poor reputation.The UN appealed last week for $459 million for immediate relief efforts and has received 40 percent of that so far, said UN spokesman Maurizio Giuliano. Another $43 million has been pledged.Aid groups have complained that the response has been too slow and not generous enough, and the UN warned that many victims have yet to receive any help. That includes around 4.6 million in eastern Punjab and southern Sindh provinces who still need shelter, Giuliano said Wednesday.The Pakistani Taliban have urged citizens to reject any foreign aid, saying it will only be stolen by the political elite in the impoverished nation of 175 million.The military, meanwhile, has some 60,000 troops dealing with flood relief. Many of those soldiers would normally would be battling insurgents or holding territory they had already cleared.US Ambassador to Pakistan Anne Patterson said Tuesday that it was too soon to understand what impact the disaster would have on the Taliban and al-Qaida in Pakistan, but that it was a concern for Washington. The US has pushed Pakistan to eliminate militant hideouts it fears are being used as rest stops for insurgents engaged in the war in Afghanistan.