  Let $A$ be a positive bounded linear operator acting on a complex Hilbert
space $\big(\mathcal{H}, \langle \cdot\mid \cdot\rangle \big)$. Let
$\omega_A(T)$ and ${\|T\|}_A$ denote the $A$-numerical radius and the
$A$-operator seminorm of an operator $T$ acting on the semi-Hilbertian space
$\big(\mathcal{H}, {\langle \cdot\mid \cdot\rangle}_A\big)$ respectively, where
${\langle x\mid y\rangle}_A := \langle Ax\mid y\rangle$ for all $x,
y\in\mathcal{H}$. In this paper, we show that
  \begin{equation*}\label{m1}
  \tfrac{1}{4}\|T^{\sharp_A} T+TT^{\sharp_A}\|_A\le \omega_A^2\left(T\right)
\le \tfrac{1}{2}\|T^{\sharp_A} T+TT^{\sharp_A}\|_A.
  \end{equation*} Here $T^{\sharp_A}$ is denoted to be a distinguished
$A$-adjoint operator of $T$. Moreover, a considerable improvement of the above
inequalities is proved. This allows to compute the $\mathbb{A}$-numerical
radius of the operator matrix $\begin{pmatrix} I&T \\ 0&-I \end{pmatrix}$ where
$\mathbb{A}= \text{diag}(A,A)$. In addition, several $A$-numerical radius
inequalities for semi-Hilbertian space operators are also established.
