Projeto web no eclipse com build.xml

Estou fazendo um teste com o build.xml no eclipse. Criei um projeto web, meus arquivos jar estão no lugar certinho na aplicação, etc, etc...porém ao executar o build quando chega no javac ele acusa

[javac] Compiling 1 source file

[javac] C:\eclipse\workspace\Web\WEB-INF\src\servlets\HTTPGetServlet.java:4: package javax.servlet does not exist

[javac] import javax.servlet.<em>;

[javac] ^

[javac] C:\eclipse\workspace\Web\WEB-INF\src\servlets\HTTPGetServlet.java:5: package javax.servlet.http does not exist

[javac] import javax.servlet.http.</em>;

[javac] ^

[javac] C:\eclipse\workspace\Web\WEB-INF\src\servlets\HTTPGetServlet.java:8: cannot resolve symbol

[javac] symbol  : class HttpServlet

[javac] location: class servlets.HTTPGetServlet

[javac] public class HTTPGetServlet extends HttpServlet {

[javac] ^

[javac] C:\eclipse\workspace\Web\WEB-INF\src\servlets\HTTPGetServlet.java:10: cannot resolve symbol

[javac] symbol  : class HttpServletRequest

[javac] location: class servlets.HTTPGetServlet

[javac] protected void doGet(HttpServletRequest request,

[javac] ^

[javac] C:\eclipse\workspace\Web\WEB-INF\src\servlets\HTTPGetServlet.java:11: cannot resolve symbol

[javac] symbol  : class HttpServletResponse

[javac] location: class servlets.HTTPGetServlet

[javac] HttpServletResponse response) throws ServletException, IOException {

[javac] ^

[javac] C:\eclipse\workspace\Web\WEB-INF\src\servlets\HTTPGetServlet.java:11: cannot resolve symbol

[javac] symbol  : class ServletException

[javac] location: class servlets.HTTPGetServlet

[javac] HttpServletResponse response) throws ServletException, IOException {

[javac] ^

[javac] 6 errors

BUILD FAILED: C:\eclipse\workspace\Web\build.xml:16: Compile failed; see the compiler error output for details.