Cannot close Oracle connection from WebSphere datasource

I try to connect to Oracle database (check connection status). I'm using following code, which works fine.

public String getDatabaseStatus() {
    Connection conn;
    try {
        conn = DriverManager.getConnection( "jdbc:oracle:thin:@192.168.0.70:1521:XE", "foo","bar");
        conn.close();
    } catch (Exception e) {
        return "ERR - " + e.getMessage();
    }
    return "Connection succesful";
}

However, when using Websphere datasource, after 10 (connection limit) refreshes page hangs. Code:

public String getDatabaseStatus() {
    Connection conn;
    try {
        Context ctx = new InitialContext();
        DataSource ds = (DataSource) ctx.lookup("jdbc/xe");
        conn = WSCallHelper.getNativeConnection(ds.getConnection());
    } catch (Exception e) {
        return "ERR - " + e.getMessage();
    }
    return "Connection succesful";
}

I tried to close provided connection, but it gives me error:

J2CA0206W - A connection error occurred. To help determine the problem, enable the Diagnose Connection Usage option on the Connection Factory or Data Source. This is the multithreaded access detection option. Alternatively check that the Database or MessageProvider is available.

Any help will be appreciated.