(CNN) -- Federal authorities warn that U.S. Web sites and e-mail servers are coming under an increasing number of attacks and that the malicious hacking could escalate in the next few days because of upcoming memorial days in China. <BR> <BR>The recent tension between the United States and China was cited by the National Infrastructure Protection Center when it issued the warning Thursday. <BR> <BR>The NIPC Web site also said Chinese hackers have publicly discussed stepping up their efforts from April 30 through May 7. That time period includes dates of historic significance to the People's Republic of China.<BR> <BR>The agency warned networks and system administrators to closely monitor their Web sites and mail services during that period. <BR> <BR>'Lion' worm linked to China<BR>Federal investigators have linked an Internet worm named "Lion" to China. The worm is infecting computers and installing denial of service tools on various systems. <BR> <BR>The worm sends password files from the victim site to an e-mail address located in China, authorities said. <BR> <BR>Some of the attacks earlier this month on U.S. sites were apparently sparked by the United States spy plane crisis in China. <BR> <BR>Hackers have showered U.S. government and business Web sites with eulogies for the downed Chinese fighter pilot, denouncements of imperialism and crude references. <BR> <BR>One of the attack victims said that vandalism appeared to have originated in China. <BR> <BR>"We discovered that the home page on one of our sites had been replaced with a posting of a Chinese flag with some rhetoric in Chinese and English," said Dan Olasin, president of Intelligent Direct. <BR> <BR>The map-selling company uses the Dell Computers site-hosting service, which helped trace the hackers to Internet Protocol (IP) addresses in China, Olasin said. Intelligent Direct has since solved the problem by having Dell rebuild its server. <BR> <BR>Hacker Union of China<BR>Hackers identifying themselves as members of the Hacker Union of China took credit for at least one attack. <BR> <BR>The defaced site was that of Iplexmarin.com, based in California. The site was covered in Chinese flags, political slogans in Chinese and English and photographs of the missing Chinese pilot Wang Wei. <BR> <BR>Wang crashed and presumably died after his fighter jet collided with a U.S. spy plane off the coast of China on April 1. <BR> <BR>"As we are Chinese, we love our motherland and its people deeply. We are so indignant about the intrusion from the imperialism. The only thing we could say is that, when we are needed, we are ready to devote anything to our motherland, even including our lives," the posting read. <BR> <BR>A Web page hosting the Hackers Union of China posted a list of 10 Web sites hacked in memory of the missing pilot. The Hackers Union refers to itself an a "network security organization" on its Web site, cnhonker.com. <BR> <BR>According to a Washington Post report, other targeted sites included two maintained by the U.S. Navy, although neither was militarily critical. <BR> <BR>Key dates: <BR>Here are dates of significance to China that authorities warn might see an increase in hacking efforts:<BR>• May 1 -- May Day, an international labor holiday.<BR>• May 4 -- Youth Day, commemorates 1919 student demonstration against "foreign aggression."<BR>• May 7 -- Anniversary of accidental NATO bombing of Chinese embassy in Belgrade, Yugoslavia